Hooter nub

Basically, a nipple.
As for the origin of it, in the anime called "No Game No Life", Sora, the main character, played a game of word chain, and he said "chikubi", which is "nipple" in Japanese, so the translators had to come up with a word that links "beach" and "bikini", and that's what they created.

At the bar:
- Damn, look at dem hooter nubs over there
- I know, mate, just leave it to me, and we'll have some threesome in no time