Corruption of the word "Twat", meaning a womans genitals, also used as an insult. Was first used to insult a Welsh teacher, who put the "-ash" suffix on every word she could, for example "Put thash pencil down!"
In a sexual manner:
"Mark would love to lick Nina's twash, but he has a bird"
To wash ones vagina after sexual intercourse. usually wiped with a wet towel to remove lube, etc. mostly referring to outside cleaning, although could also be substituted for the more vulgar word, douche.