When someone does something that is either so cool or so dumb that an uncontrollable urge arises and you smack them.
Evan: "Man, I'm going to throw this pudding at the principal."

Mitch: *Slaps Evan*

Joe: "Evan, that was slap-tastically dumb."
by Riverboy March 17, 2006
Get the slap-tastic mug.