The only free and comprehensive online etymological dictionary of the Spanish language
xeno- Prefix "foreigner"
Very early 20th cent. Borrowed from Ancient Greek ξένος ‎(ksénos) "foreign." Mycenaean ke-se-nu-wo (ksenwos). Of unknown origin, possibly borrowed from a non-Indo-European language.
"The free man, born into a group, is opposed to the stranger (Gr. xénos), that is to say, the enemy (Lat. hostis), who is liable to become my guest (Gr. xénos, Lat. hospes) or my slave if I capture him in war (Gr. aikhmálōtos, Lat. captivus)." ~ E. Benveniste, Indo-European Language and Society (1973)
ya (Adverb) "now," "already"
Very early 12th cent. From Latin iam 'id.' From Proto-Italic *iām "as far as this is concerned." From Proto-Indo-European *h1i̯-h2m "this," in the accusative case. Probably from *īm "he," "she;" the *h2 added via analogy with other pronouns.
Indo-European
Romance
Asturian , Portuguese , Galician xa, Catalan ja, French , Italian già
y, e (Conjunction) "and"
10th cent. Old Spanish e, uncommon hi, i. From Latin et 'id.,' a word in competition with suffix -que of the same meaning. Proto-Italic *eti 'id.' From Proto-Indo-European *h1eti̯ "still," "also."
Indo-European
Romance
Asturian y, Portuguese e, Galician e, Catalan i, French et, Italian e, Aromanian e, Romanian e, Sardinian e
Italic
Umbrian et 'id.,' Paelignian et 'id.'
Celtic
Gaulish eti "yet," Old Breton et- 'id.'
Germanic
Gothic "but"
Hellenic
Ancient Greek ἔτι (hépi) "still,
Indo-Iranian
Sanskrit áti "beyond," Avestan aiti 'id.'
Following de Vaan (2014), in Proto-Indo-European and later, the word meant "furthermore" to coordinate an additional phrase onto a previous statement while the language used the suffix *-kwe (Latin -que; not in Spanish). Eventually the *-h1et overshadowed and finally replaced the suffix.
yesca f. (Noun) "tinder"
13th cent. From Vulgar Latin *esca "kindling." From Latin esca "food," from esse "to eat." From Proto-Italic *ed- "to eat" (see note under comer for a continuing etymology).
Indo-European
Romance
Asturian yesca, Portuguese isca, Galician esca, Catalan esca, French esche, Italian esca, Aromanian eascã, Romanian iască
Italic
Oscan edum "to eat"
Celtic
Old Irish ithid "to eat"
Germanic
Gothic itan "to eat," Old Norse eta 'id.,' Old High German ezzan 'id.,' Old Saxon etan 'id.,' Old English etan 'id.' (English to eat)
Balto-Slavic
Old Church Slavonic jasti "to eat," Russian est' 'id.,' Czech jísti 'id.,' Polish jeść 'id.,' Slovene jẹ́sti 'id.,' Old Prussian īst 'id.,' Lithuanian ė́sti 'id.,' Latvian êst 'id."
Hellenic
Ancient Greek ἔδειν (édein) "to eat"
Armenian
Armenian owtem "to eat
Indo-Iranian
Sanskrit ádmi "I eat"
Tocharian
A nätsw- "to starve," B mätsts- 'id.'
yo (Personal Pronoun) "I"
10th cent. From Vulgar Latin eo, from Latin ego 'id.' From Proto-Italic *egō 'id.' From Proto-Indo-European *h1eǵ-(o)H. From earlier *h1eǵ- 'id.'
Indo-European
Romance
Asturian yo, Portuguse eu, Galician eu, Catalan jo, French je, Italian io, Aromanian io, Romanian eu, Sardinian eo
Indubitably, the word is especially ancient in Proto-Indo-European, at one point even labelled a "Devonion rock" of the language by Benjamin Fortson (2011). Because of its extreme age, the semantic sense of *h1eǵ- and its many suffixes (Latin points to *-(o)H; Sanskrit to *-om; Germanic *-Hom; and Hittite to a word without a suffix) have inspired significant discussion (see Sihler 2008).
yugo m. (Noun) "yoke"
13th cent. Old Spanish yugo, iuuo. Borrowed from Latin iugum 'id.' From Proto-Italic *jugo- 'id.' From Proto-Indo-European *i̯ug-o- "yoke," but more literally "joined together." From the root *i̯eu̯g- "to yoke together."
Also the origin of the surname Yugo.
Variants
Almerían huvo, Old Spanish iuuo
Indo-European
Romance
Asturian xugu, Portuguese jugo, Galician xugo, Catalan jou, French joug, Italian giogo, Aromanian chubo, Romanian jug, Sardinian giú
Celtic
Old Irish cuing "yoke," Middle Welsh iou 'id.,' Middle Breton yeu 'id.,' Cornish ieu 'id.'
Germanic
Gothic juk "yoke," Old Norse ok 'id.,' Old High German joh 'id.,' Old Saxon juk 'id.,' Old English geoc 'id.' (English yoke)
Balto-Slavic
Old Church Slavonic igo "yoke," Russian ígo 'id.,' Czech jho 'id.,' Polish jugo 'id.,' Slovene igọ̑ 'id.,' Lithuanian jùngas 'id.,' Latvian jûgs 'id.'
Indo-Iranian
Sanskrit yúj- "ally," Young Avestan yuiiō- "yoke"
Myriad written attestation in Spanish and in Romance languages affirm that this is not a learned borrowing from Latin, yet Corominas (1991) notes that we should expect **jogo from iugum, and not yugo. Preservation of yu- could be from Leonese influence.

Old Spanish iuuo (early 13th cent.), however surprising, cannot be dismissed as a writing error: in the Almerían dialect we find a modern reflex in huvo. This must be a continuation of Vulgar Latin *iuum.
The ability to reconstruct a word for yoke in Proto-Indo-European proves that the ancient Indo-Europeans had draft animals used to pull wheeled carts.
zacate f. (Noun) (Latin America) "grass"
16th cent. Borrowed from Nahuatl zacatl 'id.'
The word is also found in the surnames Zacate, the conservative form Zácatl, Zacatzi (with the Nahuatl honorific suffix -zi), and Zacateco (with the honorific suffix -co). Also found in the surname Zacatenco "on the grassy coast," from Nahuatl zaca (apocopation of zacatl) and ten "coast."
zaga f. (Noun) "rear"
12th cent. From Andalusian Arabic sáqa 'id.' From Arabic sāqah "rearguard."
Variants
Andalusia azagador (trashumance) "royal path for cattle," azagar "to drive cattle in a line," probably borrowed from Aragonese azagar
Indo-European
Romance
Aragonese azagar "to drive cattle in a line," Catalan assagador "farmer's path"
The word is not the origin of the surname Zaga. The surname, according to Tibón (1988), was originally given to Sephardic Jewish families and is of unknown origin. Tibón speculates the surname may derive as an apocopation of Zagarolo, name of a town near Rome, Italy.
zanahoria f. (Noun) "carrot"
14th cent. Old Spanish çanahoria, dialectic variant safanòria. Borrowed from Andalusian Arabic *safunnárya 'id.,' itself borrowed from Ancient Greek σταφυλίνη ἀγρία (staphulíne agría) "wild carrot." Σταφυλίνη "carrot" is a diminutive of σταφυλή (staphulé) "grape," of unknown origin. Ἀγρία, when referring to plants meaning "wild," is from ἀγρός (agrós) "field" (see agro).
Indo-European
Romance
Catalan (Mallorca) safanària
Basque
Gizpuzkoan azenario "carrot," borrowed from Old Spanish
zapato m. (Noun) "shoe"
10th cent. Old Spanish çapato. Of unknown origin but it must be a borrowing from another language. In languages of the Iberian peninsula and in Italian, the meaning is of shoes, but in Gallo-Romance the sense is pejorative.
Zapato is the origin of the surnames Zapata, Zabata, Zapato, Zapatel, and Zapatero. The surnames Sabada and Sabadanes spring from Old Spanish çapato. All of these surnames were originally nicknames given to cobblers.
Indo-European
Romance
Portuguese sapato, Italian ciabatta
"Una palabra semejante existe en las lenguas eslavas del Norte ..., en turco septentrional ..., y alguna forma semejante se ha empleada en persa, pero no es seguro que haya relación etimológica entre estas palabras orientales y las lenguas de Occidente; si hubo propagación de las unas a las otras, no consta el lugar de origen; la documentación más antigua que hasta ahora se ha encontrado procede de España cristiana y de la parte musulmana del mismo país, y en ninguna parte se encuentra una etimología que se imponga por razones lingüísticas." ~ J. Corominas, Diccionario crítico etimológico castellano e hispánico, (1991).