Etymology
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lagoon (n.)

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]
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atoll (n.)
"island consisting of a strip or ring of coral around a central lagoon," 1620s, atollon, from Malayalam (Dravidian) atolu "reef," which is said to be from adal "closing, uniting." Watkins writes, "Perhaps ultimately from Sanskrit antara-, interior" (from PIE root *en "in"). The original use was in reference to the Maldives. Popularized in present form by Darwin's writings.
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haff (n.)

also haaf, Baltic lagoon, separated from open sea by a sandbar, German, from Middle Low German haf "sea," from Proto-Germanic *hafan (source also of Old Norse haf, Swedish haf "the sea," especially "the high sea," Danish hav, Old Frisian hef, Old English hæf "sea"), perhaps literally "the rising one," and related to the root of heave, or a substratum word from the pre-Indo-European inhabitants of the coastal regions. The same word as haaf "the deep sea," which survived in the fishing communities of the Shetland and Orkney islands.

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