Hideously profane Japanese word meaning "vagina." It is never written out in manga or books: instead, they censor the middle "n" with a maru, so it comes out as ま〇こ.

A quick story illustrates the potency of this word. One of my friends was having a deep intellectual conversation with some schoolgirls, and they began comparing English and Japanese. The schoolgirls were arguing that there was no profanity in Japanese whatsoever. My friend mentioned "manko" to them, and they all recoiled in fright. (It was a hilarious sight, by the way. Try it sometime.)

Seriously, though, manko is never heard in female conversation (or at least nobody will admit it), and it is only rarely heard in the roughest male conversation. Asoko ("over there") is usually the preferred euphemism to avoid mass panic.

In the Kansai dialect, the word meko is used in the same way.