"Shaun Van Poecke" wrote
Come on Ellen, you saw that in 'boondock saints', the reply to which was 'maybe it should have been rule of wrist' and
im not sure if it was a true story or not :P

Never heard of it.... But, here's a google search find:
http://tafkac.org/language/etymology/rule_of_thumb.html

Judge Thumb... http://www.worldwidewords.org/qa/qa-rul1.htm

Cheers,
Ellen