The only free and comprehensive online etymological dictionary of the Spanish language
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)