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.
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)