I understand that the above rejoinder is an expression of disbelief, that you think someone is taking you for being a gullible clod. But what the heck does it mean? Cursory web searches haven’t helped me nail this one down. Explanations like “it means they’re pulling your leg” or “it is derived from the days when they hung people” don’t clarify it for me. I scanned a dozen threads in SDMB without achieving further clarity. Sounds vaguely british. Any dopers able to enlighten me?
Morris dancers with bells strapped to their legs?
Google led me to this;
http://www.shu.ac.uk/web-admin/phrases/bulletin_board/1/messages/2674.html
Scottish, to trip someone to make them look foolish.
Could be.
Peace,
mangeorge
So the village idiot had bells on one leg; therefore “you think I’m so gullible as to believe that, you must take me for the village idiot who has bells on one leg”?
Possible, I suppose. Any other hypotheses?
Cassell’s Dictionary Of Slang fails to give an origin but does date the expression from the 1960’s.
Well no, Qadgop, not exasctly. I think your phrase is more like saying “Yeah, right. Pull my other leg, at least you might hear bells ring”. The ‘bells’ part is probably a later addition.
Or is that what you said?
Peace,
mangeorge
Bells are a standard addition to the costume of a medieval court jester. Thus, “Pull the other one, son, it’s got bells on.”
Used to convey disbelief, i.e. ‘Do you take me for a fool?’ 
–Nut
Mebbe so, but I was 15 in '60 and I’m sure I heard it before that.
'Course I can barely remember my phone #.
So who you gonna believe, me or some old fart named “Cassell”? 
Peace,
mangeorge
So THAT’S what they say in Monty Python’s SFTHG!
At the begining…I could never figure that line out.
Thanks all, especially to nutwrench whose reply so far makes the most (if not a lot of) sense.
The bells were sewn onto a garter which was worn around one ankle by the court jester. The other leg didn’t have any.