(v). to slap the cactus, to be slapping the cactus, to have slapped the cactus.
A phrase used upon the realization that you are (or even within the act of) doing something worthy of a *facepalm* only it is worse. It is also the more exciting substitute for calling someone dumb, or stupid.
"Sarah was definitely slapped the cactus last night - she turned the stove on, forgot that she had, and proceeded to set her laundry down - thankfully she only singed her eyebrows when she tried to put the fire out with the contents of a suspicious water bottle that turned out to be full of vodka"
Sam: "Come on man, you HAVE to admit, Shelby is pretty hott"
Tim: "Dude, stop slapping the cactus - she looks like a mule"