1670s, lagune, earlier laguna (1610s), "area of marsh or shallow, brackish water beside a sea but separated from it by dunes," from French lagune or directly from Italian laguna "pond, lake," from Latin lacuna "pond, hole," from lacus "pond" (see lake (n.1)). Originally in reference to the region of Venice. The word was applied 1769 (by Capt. Cook) to the lake-like stretch of water enclosed in a South Seas atoll. Also see -oon. Related: Lagoonal.
In regions where Spanish is or formerly was the current language, the word lagoon is likely to be used with more latitude of meaning, since in the Spanish laguna is applied to ordinary lakes, to the bottoms of deep bays, especially when these are more or less closed in by a narrowing of the coast-lines, so as to give rise to lake-like areas, and also to shallow, swampy, or almost dried-up lakes inland as well as near the coast. [Century Dictionary]
in poetic use, "a swallow" ("But the poets appear to have thought it some song-bird" - OED), late 14c., also Progne, from Latin Procne, Progne, from Greek Proknē, in mythology the name of the daughter of Pandion and sister of Philomela, who was transformed into a swallow.
Proto-Indo-European root meaning "body of water."
It forms all or part of: aquamarine; Armorica; beche-de-mer; cormorant; mare (n.2) "broad, dark areas of the moon;" marina; marinate; marine; mariner; maritime; marsh; mere (n.1) "lake, pool;" Merlin; mermaid; merman; meerschaum; meerkat; morass; Muriel; rosemary; submarine; ultramarine; Weimar.
It is the hypothetical source of/evidence for its existence is provided by: Latin mare; Old Church Slavonic morje, Russian more, Lithuanian marės, Old Irish muir, Welsh mor "sea;" Old English mere "sea, ocean; lake, pool," German Meer "sea."