"Pulling someone's leg," meaning to spoof someone with a lighthearted gag, is a good example of a colloquial idiom, a popular saying which means more than just the words it contains. If I say that my friend is "a loose cannon," for instance, I am saying that he is unpredictable, not that he is an artillery piece. In the case of "loose cannon," it's fairly easy to pinpoint the literal source of the metaphor: an untethered cannon rolling around on a ship's deck is unpredictable and possibly disastrous. But in the case of "pulling your leg," we have a genuine mystery.

Not that there is any lack of theories, of course. Perhaps the most commonly heard is that the phrase refers to pulling on the legs of someone being hanged in order to speed up the process. The phrase, however, dates from about 1888, long after the technology of hanging had rendered such grisly embellishments unnecessary. A more likely source is the practice of street thieves tripping their victims as a prelude to robbing them. To "pull someone's leg" thus meant to trick, disorient and confuse a person, a meaning which lives on today in our more benign "just kidding" sense.