Generally a word subtly replaced with "what" when someone you don't like says something to you from a distance or you can't understand what they said.
You can call them a twat without them knowing and silently giggle to yourself.
"Hey John! GET BUSY OR PRETEND LIKE YOU ARE!"
John replies like he didn't hear him
"I cunt hear you"
In most cases the meaning of the original thought when rendered as a thwat is compromised.
As with a meme (an idea denuded of meaning) a thwat might be thought of as thought without the thinking.