A chick who is decent enough looking to bag, but only after several beers, is said to have a "box of arms" because of all the guys who have had those beers, done the deed, and woken up on the next morning beside her and thought "shit, i gotta get out of here bfore she wakes up". But she's laying on his arm--so he has to knaw it off. So, over time, the woman has all these guys arms, hence she has a 'box of arms'
by Big Hammer December 27, 2012