My guess is that in this context it means "to beat somebody up".

A "deck" is the floor of a ship. "To deck someone" means you make them fall to the floor(deck) by punching them. it's common in Australia as well. Do not confuse this with "to dack someone". :)

It is a UK slang term. Deck = Punch or hit. e.g. "He decked the guy with one punch", "shut up or I'll deck you"

punch somebody to the ground

Oh I see. THX!