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.
A particularly prized
comic book,
sheathed in two
layers of the finest
Mylar.
A Message To You Fanboy