A term applied to girls that look/dress/act older than their age who should be forced to wear a wristband (like the ones given out at clubs) so you can tell whether they are over 18 or not.
Used either as a warning from one man to another that the girl may be underage
Will: Check out the booty on that chick!
Matt: I dunno man, that one needs a wristband
or as an exclamation to draw attention to the girl.
(Hot, Young Girl Walks By)