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1.
(n)-Used to describe any sort of deeply embarrassing life-affecting truth or habit that one attempts to keep secret. In a literal sense, it refers to one keeping a bag of one's hair, toe nail clippings, or other strange items over a long time.
Boyfriend: "By the way, I've been meaning to tell you something, and I'm not sure how you'll take to it..."

Girlfriend: "Oh boy, here comes the bag of hair..."
by Coquigamer December 10, 2005
 
2.
what dumb people can be compared to...along the lines of "box of rocks" or "bag of hammers"
1st chick :"She is stupider than a bag of hair"
2nd chick:" Stupider???"
by hamina hamina hamina September 15, 2003