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