To pull someone's leg - Idiom of the Week
As defined in Linda and Roger Flavell’s book “Dictionary of Idioms and Their Origins,” “to pull someone’s leg” is an idiom that means “to make someone the target of a good-humored joke or deception” (page 184).
Aw man! You didn’t think I was serious when I said you needed to stay at work late on Friday, did you?! I was just pulling your leg!
The origin of this idiom is somewhat disturbing. Please note that the modern meaning of the idiom is not at all used or related to the ancient origin, which is more violent in nature.
It has been said that this idiom refers to a criminal who has been sentenced to death by hanging. His or her relatives had the right to pull or tug on their legs, so as to speed up their death, mercifully.
Because of this dark origin, some find it tough to make the connection between this gruesome history and the light, humorous modern sense of the expression.
After the dark ages, when this violent meaning became less relevant, it was more likely to be used in the sense of pulling one’s leg out from under them, or tripping them in public. This would make the individual a subject of public ridicule.
While this second derivation isn’t very upbeat or light-hearted either, it is much easier to tie to the modern day meaning of making one the subject or target of a light-hearted, good-natured joke.