In certain knuckle-dragging male circles (eg. some college fraternities), an especially unattractive chick.

In order that you can avoid looking at her ugly face she wears a bag over her head. To avoid the ridicule of your peers and the shame and stigma of being seen with such a disgusting date, you wear a bag over yours. Simple.

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A particularly prized comic book, sheathed in two layers of the finest Mylar.

A Message To You Fanboy

I always thought that a double-bagger was a women so ugly that not only were you supposed to put a bag on her head before having sex with her. You made sure that she was actually wearing two bags, you know, in case one of them accidentally fell off.

Also see: Ugly From the Front and Good from far, but far from good