The only free and comprehensive online etymological dictionary of the Spanish language
la f. (Definite Article) "the"
From Latin illa "that." Old Latin olla. From Proto-Italic *olna 'id.' From Proto-Indo-European *h2ol-na- "that." From the root *h2el- "that." See also ella.
Indo-European
Romance
Asturian la, Portuguese la, Catalan la, French la, Italian la, Romanian aia
labor m. (Noun) "labor"
11th cent. Borrowed from Latin labor 'id.' Of unknown origin.
Indo-European
Romance
Asturian llabor, Portuguese labor, French labeur, Italian lavoro, Romanian laboare
laborar (Verb) "to labor;" "to conspire"
19th cent. Borrowed from Latin laborare "to labor," a verb formed from labor "labor" (see labor).
Indo-European
Romance
Asturian llabrar, Portuguese laborar, Galician labrar, Catalan laborar, French labourer, Italian lavorare
laboratorio m. (Noun) "laboratory"
18th cent. Borrowed from Medieval Latin laboratorium 'id.' From Late Latin laborator "laborer," from laborare "to labor" (see laborar).
Indo-European
Romance
Portuguese laboratório, French laboratoire, Italian laboratorio
lado m. (Noun) "side"
13th cent. From Latin latus 'id.' From Proto-Italic *slāto- 'id.' Of disputed origin.
Also the origin of the surname Lado.
Indo-European
Romance
Portuguese lato, Catalan lat, Italian lato, Aromanian lat, Romanian lat
Celtic
Old Irish con·slá? "to go away," a connection posited by de Vaan (2014), but he has not convinced most authorities due to semantic problems. It is hard to explain the connection between "going away" and "side."
Balto-Slavic
Old Church Slavonic stьlati? "broaden." Classically linked to Latin latus, but now reconstructed in Proto-Indo-European without a laryngeal as *stel- "to place" (Rix, et al. 2001).
lagar m. (Noun) "wine press"
11th cent. From lago and the suffix -ar (2), indicating a place.
lago m. (Noun) "lake"
Early 13th cent. From Latin lacus 'id.' From Proto-Italic *laku- 'id.' From Proto-Indo-European *lok-u̯ 'id.' The form is difficult, especially with evidence of a zero-grade in Greek λάκκος (lákkos) "pond," "cistern." To add to the problem, the word looks like a u̯-stem in Proto-Indo-European, yet Proto-Hellenic *-kw- shouldn't produce geminate -κκ-.

The word is probably a late Proto-Indo-European development or Greek borrowed the word from another Indo-European language. An alternative theory that the word represents an old a-root ablaut (see below) raises more phonological problems than it solves.
Also the origin of the surname del Lago, Lagos and Laguillo.
Indo-European
Romance
Asturian llagu, Portuguese lago, French lac, Italian lago, Aromanian lac, Romanian lac, Sardinian lagu
Celtic
Old Irish loch "lake"
Germanic
Old Norse lǫgr "sea," Old High German lahha "puddle," Old Saxon lagu "lake," Old English lagu 'id.'
Balto-Slavic
Old Church Slavonic lokъvi "puddle," BCS lȍkva 'id.'
Hellenic
Ancient Greek λάκκος (lákkos) "lake"
Basque
Basque lak(h)u "lake" (borrowed from Latin)
As mentioned above, the word lago may in fact be a remnant of a Proto-Indo-European ablaut with a root-vowel *a. This is controversial at best, and not reflected in the etymologies in our dictionary, however the bare possibility demands mentioning.

"Although the evidence is sparse, it appears that roots with a as fundamental vowel also ablauted. The root *sal- 'salt' had a zero-grad *sl̥-...; the root *nas- 'nose' has a lengthened-grade derivatives such as Latin nār-ēs and English nose, both from *nās-; and the root *laku- 'body of water' (Lat. lacus 'lake', Gk. lákkos 'pond') had an o-grade form *loku- that became Scottish Gaelic loch 'lake'. The view that roots in a ablauted is not universally accepted, but these forms are difficult to explain otherwise." ~ B. Fortson, Indo-European Language and Culture (2011)
laguna f. (Noun) "lagoon"
11th cent. From Latin lacuna 'id.,' a diminutive of lacus "lake" (see lago).
Indo-European
Romance
Portuguese lagoa, Galician lagoa, Italian laguna
laja f. (Noun) "leash"
16th cent. From Latin laxa "wide," "loose." From Proto-Italic *slaksā-, masculine *slakso- 'id.' From Proto-Indo-European *sl̥ǵ-so- 'id.'
Indo-European
Romance
Italian lasso
Italic
Latin languere "to be faint" (from *(s)l-n̥-ǵ-u̯- "faint")
Germanic
Old Norse slakr "weak," Old High German slach 'id.,' Old Saxon slak 'id.,' Old English slæc 'id.' (English slack)
Hellenic
Ancient Greek λάγνος (lágnos) "voluptuous"
Indo-Iranian
Sanskrit ślakṣṇá- "slippery"
Tocharian
A slākkär "sad," B slakkare "darting"
lamentar (Verb) "to be sorry," "to lament"
15th cent. From lamentare 'id.,' from lamentum "lament" (see lamento).
Indo-European
Romance
Portuguese lamentar