I read it, but they didn't cover leg-pulling. So I searched it online. I know I found a few theories as to what the origin of that phrase was, but it apparently didn't leave a lasting impression on me, because I've completely forgotten it now.
I did start wondering about some other common phrases, and who determined them. For example, (and yes, sadly, this is one I spend A LOT of time thinking about) "Rat's A**". For the sake of my family friendly blog, we're going to simply refer to this as Rat's Bum from now on. Who determined that a rat's bum was the lowest form of universal currency in the world. When you can't even be bothered to "give a rat's bum" about something... that shows that it is REALLY far down. How do you suppose that was used for the first time? A bunch of people sitting around with excess rodent parts, and they determined that the rat's bum was worth the least of all other animals and their various parts? I think it would be a little more disturbing if you could give a rat's bum about something. Seriously? Where would you keep those? Would you have a box of rat bums? I can just imagine that somewhere out there is some sicko just waiting for the perfect opportunity for someone to say to him/her, "I couldn't give a rat's bum" and then they say, "Well, I could!" And out of the box comes the rat's bum.
And also, who decided that a horse's bum was equivalent to the worst class of people? As a devoted horse-person, I find that one kind of offensive. I actually made a friend apologize to a picture of Shadow once after referring to someone else that we know as a horse's bum. I said that was an insult to my horse and his bum. Why a horse? What makes a horse's bum worse than any other animals? Although, clearly it's still worth more than a rat's, it's still looked down upon in social circles.
Who decided that chickens were more cowardly than any other bird? Why do we call people "chicken?" I think that any bird who brazenly crosses the road regardless of speeding vehicles is not inherently cowardly. Stupid? Heck yes. Filthy? Absolutely! Delicious? Why yes, yes they are! But cowardly? I'm not so sure.