Ultimately I'd say it would come down to DM fiat/fluff.
If you look up the definition of slam you get "shut (a door, window, or lid) forcefully and loudly". Though it can be a very generalised term.
It could be a full body slam as you suggest i.e "the zombie sort of trips and slams into you, dealing X damage" or some form of graceful, acrobatic attempt, it could be an "amateur trying to break through a door" shoulder slam, it could be an unarmed, two-handed, closed-fist bash as @Key suggests or a kick for that matter, if the monster is actually holding a weapon they could slam the pummel into you, they could just pick up a rock and slam that into the side of your head.
It really kind of depends on the situation, the monster and how the DM wants to describe it.