This is a British phase and is used in the past tense, it means that one beat-up another, however it is also used to descirbe a single yet dramatic hit.
was all up in Dean's face, so he twatted him, right between the eyes"
This is was orginally a British phase meaning one is either extremely intoxicated or angry, the latter has been adopted by Americans.
Drunk: "I got so pissed in town last night, Satan's Hallow FTW
Angry: "Sorry about last night, i was so pissed-off
at my parents"