It's when your holding a cat and it wriggles and you sort of just, thumb the cat.
Gavin:So I was holding Geoff's cat and it wriggled and I, Uhh
Geoff:He fisted my cats anus
Gavin:I cant beleive I had a slip o' the thumbs
When you're holding a cat, or any small animal, and it backs onto your thumb, causing you to penetrate the animal.
Gavin had a Slip O' The Thumbs when he was holding Geoff's cat.