lagoon

Borrowed from French lagune, itself from Italian laguna, which traces back to Latin lacūna. It's also distantly related to Old English lagu through a shared Proto-Indo-European root.

The facts given don't describe a change in meaning — only the path it took through French and Italian back to Latin.

It's a doublet of lacuna and lacune, meaning all three English words descend from the same Latin ancestor, lacūna, just by different routes.