you initiate a "Donkey Punch", you realize she hasn't gone down for the count, so you immediately break out your trusty chainsaw, cut down the nearest, beefiest, piece of American lumber you can get your naughty mitts on, put on your favorite flannel shirt, get back on your saddle, tell your lady friend you're sorry, and right when she's about to climax, you ring her melon with your new found buddy, toss it over your shoulder and step on her back in victory.
Ahhh yes im gonna tap your ass till it.... ooooh yes yes !!! taste my lumber bitch. POW!
Another term for cock slap, in which one takes ones penis and slaps the fce of someone who displeases them in some way with thier penis. By saying "lumberjack slap" one can insinuate a cock slap without using the harsher description.