LAKE-OON

The word lagoon first started showing up in English in the early seventeenth century with the spelling laguna. At the time, it exclusively referred to the body of water that Venice was in, but it came to be applied to other brackish lakes separated from the sea by barriers after James Cook used it to describe a stretch of water in a southern Pacific atoll. The word, possibly through French, comes from the Italian word for "pond", and the -oon suffix was added to show the stress on a borrowed noun (see buffoon, lampoon, and harpoon for similar examples). Finally, the Italian word comes from Latin lacus, which meant"pond" and is also the source of "lake", and that's from Proto-Indo-European laku​, meaning "body of water" more generally.