We thought this was funny…
 
In the last episode of Free! - Eternal Summer -, Momo excitedly demands a souvenir from Rin, who gives him a boomerang. A boomerang from Australia, that makes sense, right?
 
In some areas, including Japan, “boomerang” is actually a term for the tight brief style of men’s swim trunks—what we might colloquially refer to as a “speedo” (although Speedo is technically the name of a brand and not a specific swimsuit style). Momo’s famous short short swim shorts are a “boomerang.” In a season 1 next episode preview, Rin calls Rei “boomerang megane” or “speedo glasses” because those were his first identifying features. So this gift to Momo may be a bit of a joke, although it’s… unclear if Momo gets it.
 
~The more you know~