A one-piece bathing (or tanning) suit for women:

1. just a bottom piece, hence a euphemism for topless
2. a bikini whose top and bottom pieces are connected by cloth either a) on the front, b) the back or c) both, but with the kidney area uncovered
3. much like #2 but with a neckholder and no strings on the back
4. jokingly for a classic swimsuit which covers much of the torso or even for a burkini

This can lead to misunderstandings, obviously.

A casual invitation / threat used by an individual that is not afraid of fucking someone up.

A bathing suit for women without a top, it's generally used for tanning purposes.

1. A tongue-in-cheek term for a bikini with only the bottoms being worn.

2. A Chinese-made one-piece bathing suit, typically purchased for around $10 on eBay, Amazon, or any number of other sites that facilitate mail-order purchases from China. For whatever reason, Chinese vendors seem to believe that "monokini" is the proper term for a one-piece bathing suit, and use it extensively in item descriptions. When this definition is used, "monokini" is pronounced as though it were a Japanese word, to distinguish it from the other definition. Why Japanese, when this is a Chinese phenomenon? Probably because they're both Engrish parent languages, and "monokini" looks like a Japanese word?

the bottom half of a woman's bikini. It's more accurate to say that a a girl is wearing a monokini if she's going topless.