Noun

1. A vagina, pussy, vulva, clitoris
2. A contemptible and stupid person, idiot .
3. (Famously mistaken by Robert Browning to mean) a nun's headpiece.
   • 1841, Robert Browning, Pippa Passes,

     Then, owls and bats, cowls and twats,

     Monks and nuns, in a cloister’s moods,

     Adjourn to the oak-stump pantry!

Verb

1. To hit, slap.
   • 1989, Red Dwarf, series III, episode 3 (Polymorph)

     Arnold Rimmer: What are we gonna do?

     Dave Lister: Well, I say let's get out there and twat it!

   • 2005, S. J. Smith, Joe Public

     "Umbridge'll twat him into next week."

   • 2006: Martyn J. Pass, Dani Pass, Waiting for Red

     "I'd love to twat her over the head with this. What d'ya reckon?"

     "She'd certainly bleed a lot."

   • 2007: Mark Button, Security Officers And Policing: Powers, Culture and Control in the Governance of Private Space

     That's the only time I have been twatted by someone.