Deceiving you in a humorous or playful way.
“You’re pulling my leg” was a common slang expressions in the 20th century, and means misleading or teasing in a mild and benign way. But, like those other “leg” phrases:
— an arm and a leg
— shake a leg
— break a leg
there’s no evidence to show that the leg was anything but imaginary.
Like many phrases, it seems to be an orphan. No one knows where it originated. There are two popular stories claiming to explain it: holding onto the legs of those being executed by suspension hanging, to add weight and give them a quicker death; and thieves tripping people to rob them. Neither story has validity and neither story implies humor of any kind.
The date of origin is likely around 1880. The earliest example in print is from the Ohio newspaper The Newark Daily Advocate, February, 1883:
It is now the correct thing to say that a man who has been telling you preposterous lies has been ‘pulling your leg.’
The fact that the phrase needed to be explained to the paper’s readers is clear evidence that it was new in 1883, so if that example isn’t the earliest use, it must be close to it.
Though there’s no validity to the idea that the phrase arose from thieves tripping people in order to rob them, it might be related to the idea of physically tripping up another person, thus putting him off balance and possibly colliding with others, so that he is made to look foolish. It’s simple to extend that to mental foolery. Making someone believe (and possibly act on) a wild story will also make a person look foolish.
The British use an extended version, “pull the other one, it’s got bells on.” That means “don’t try and fool me with that crazy story, I’m not that stupid.” It has been in use in Britain since at least the 1960s, which is when the first appearances in print are found. The “bells” may be a reference to a jester’s costume.
Since my mother came from England, I grew up hearing that phrase, though it was usually shortened to, “Go on, pull the other one,” as a response to what sounded like an unbelievable tale.
So, go on, pull a leg or two; the day is yet young!