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.

Romance: Western Vulgar Latin: Asturian la, Portuguese la, Catalan la, French la, Italian la; Eastern Vulgar Latin: Romanian aia>
labor m. (Noun) "labor"

11th cent. Borrowed from Latin labor 'id.' Of unknown origin.

Romance: Western Vulgar Latin: Asturian llabor, Portuguese labor, French labeur, Italian lavoro; Eastern Vulgar Latin: Romanian laboare
laborar (Verb) "to labor;" "to conspire"

19th cent. Borrowed from Latin laborare "to labor," a verb formed from labor "labor" (see labor).

Romance: Western Vulgar Latin: 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).

Romance: Western Vulgar Latin: 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.

Romance: Western Vulgar Latin: Portuguese lato, Catalan lat, Italian lato; Eastern Vulgar Latin: Aromanian lat, Romanian lat

Indo-European: Celtic: Old Irish con·slá? "to go away" (a connection held by de Vaan (2014), but has not convinced most authorities on semantic grounds - the semantic gap between 'going away' and 'side' is more of a cavernous abyss than a small hurdle); 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.

Romance: Western Vulgar Latin: Asturian llagu, Portuguese lago, French lac, Italian lago; Eastern Vulgar Latin: Aromanian lac, Romanian lac; Sardinian: lagu

Indo-European: 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"

Note: 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).

Romance: Western Vulgar Latin: 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.'

Romance: Western Vulgar Latin: Italian lasso

Italic: Latin languere "to be faint" (< *(s)l-n̥-ǵ-u̯- "faint")

Indo-European: 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).

Romance: Western Vulgar Latin: Portuguese lamentar