The only free and comprehensive online etymological dictionary of the Spanish language
jamás (Adverb) "never"

12th cent. Ultimately from Latin iam magis "yet more" but the exact route is unclear. Probably borrowed from Old Occitan ja mais (Modern Occitan jamai). For the etymology of iam, see ya; for the etymology of magis, see más.

Romance: Western Vulgar Latin: Portuguese jamais, Italian giammai
Jano m. (Noun) Janus

From Latin Ianus 'id.' The Roman god of beginnings, archways, passages, time, and duality. He is depicted having two faces: one looking to the future and the other to the past. The origin of his name is simple, taken from Latin ianus "archway." From Proto-Italic *jānu- "door." From Proto-Indo-European *i̯eh2-n-u- "corridor" (an n-derivation of earlier *i̯eh2- "to go").

Indo-European: Tocharian: B yoñiya "path"
jefe (Noun) "boss;" "chief"

17th cent. Old Spanish xefe. From French chef, from Vulgar Latin *capum, from Latin caput "head" (see cabo).

The -e ending was originally neuter and then re-analyzed as masculine. Thus modern feminine forms are split between la jefe and la jefa. Jefa is first attested in the 19th cent.

Romance: Western Vulgar Latin: Asturian cabu, Portuguese cabo, Galician cabo, Catalan cap, French chef, Italian capo; Eastern Vulgar Latin: Aromanian cap, Romanian cap
Jesús (Personal Name) "Jesus"

From Latin Iesus, from Greek Ἰησοῦς (iesoûs), from Hebrew yeshúa. A syncopation of y'hoshúa "Joshua" in the belief that yeshúa was the original name of Jesus. From Yeho, a reference to Yahweh, and y-š-ʕ "to liberate." According to Huehnegard (2011), Yahweh historically meant "he sends down (the hosts of Heaven)" and is from a causitive stem of hāwâ "to happen."

The name is common as first and last names.

Romance: Western Vulgar Latin: Asturian Xesús, Portuguese Jesus, Galician Xesús, Catalan Jesús, French Jésus, Italian Gesù; Sardinian: Gesùs
joder (Verb) (obscene) "to fuck"

14th cent. Old Spanish foder. Study of the word is hampered by the fact that, due to its vulgarity, most authorities passed over joder without entry until the late 20th cent. It is even omitted from Corominas' Breve diccionario (1987). From Latin futuere 'id.' Of unknown origin. Possibly from Proto-Italic *fūt- "to hit" or from a noun *futu- "ability to ejaculate." See de Vaan (2014) for discussion.

Also shortened variant jo via taboo deformation, however see also jo.

Romance: Western Vulgar Latin: Asturian foder, Portuguese foder, Galician foder, Catalan fotre, Italian fottere; Eastern Vulgar Latin: Aromanian fut, Romanian fute; Sardinian: fútere
jodido (Adjective) (obscene) "fucked"

14th cent. From the past participle of joder.
jo, so (Interjection) "darn!"

15th cent. Old Spanish xo. Originally an equestrian term to call one's horse to halt. Probably of expressive origin.
jota (1) f. (Noun) "a type of bean soup"

Borrowed from Old French jotte 'id.,' from Late Latin iotta "soup." Borrowed from an Insular Celtic language (compare Old Cornish and Old Welsh iot). From Proto-Celtic *yuto- "porridge." Of unknown origin. Presumably borrowed from a non-Indo-European language.

Celtic: Goidelic: Old Irish íth "porridge;" Brythonic: Old Welsh iot "porridge," Middle Breton yot 'id.' Old Cornish iot 'id.'
jota (2) f. (Noun) "type of dance popular Aragon"

Old Spanish xota. From Mozarabic *šáwta "dance," "jump." Borrowed from Latin saltare "to jump" (see saltar).
jote m. (Adjective, Noun) (South America) "philandering;" "vulture"

Of unknown origin.