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1.
When a guy and girl hook up but there's no emotional attachment on his part, so he may as well be using his hand, but is too lazy/ it feels better with the ho.
Ben: "Hey Nick, are you and Sarah together now?"
Nick: "Nahh mate, she was just a twat wank if I'm honest..."

OR

"I'm so f**king horny, but I cba to DIY...I'm gonna call Molly for a twat wank"
by RepisShredders April 16, 2010
 
2.
A twatwank is much the same as a titwank, except it utilises a lady's tuppy. Essentially, it's just plain old sexing.
"My, Lady Chatterley, I wouldn't half love to give you a good twatwank up against that old oak tree yonder..."

"The twins, Alan and Victoria, slipped into the train's toilet with a view to steal a quick kiss from one another. As it turned out, Alan ended up furiously twatwanking Victoria up against the door."
by Ian Gee April 21, 2006