Twat is a vulgar word roughly equivalent to vulva.

Used by poet Robert Browning in his poem Pippa Passes as an article of ecclesiastical clothing. See, Browning had seen a quote from 1656, a little doggerel that included:

They talk't of his having a Cardinalls Hat,
They'd send him as soon an Old Nuns Twat.

and figured it must be some kind part of a nun's habit. Oops.