From: Samuel Barringer
On "Mythbusters," Tory, Kari and Grant tried to see if a punch or some other force could literally knock one's socks off. The force from the explosion of several hundred pounds of explosives knocked the socks off some mannequins. The only problem was the force was considered lethal, and with the other experiments that failed to knock off any socks, the myth about knock your socks off was busted. I am not sure why any one would use that idiom when it is not even true to begin with.