1) A contraction of two words ("smack" and "down") popularized by WWE wrestler-turned-actor The Rock that refers to the inflicting of a beatdown of epic proportions upon "jabroni"s' "candyasses". Sometimes used in a bastardized form of Elizabethan English.
2) The name of the Thursday night WWE show on UPN created by Vince McMahon originally as a show of respect for his champion, The Rock, who commonly used the term. Now it is merely a place where stars' careers go to die.
1a) The Rock prepares to lay the smackdown on Triple H.
1b) Know your role and shut your mouth before I layeth the smacketh-down on ALL your candyasses!
2) Chris Benoit has left SmackDown! for RAW to finally earn some redemption for his career.