Adjective: used to descibe a broad who is so old the hairs on her BOX (synonym for vagina; see VAGINA) have turned GREY....
Server #1 "Hey, is your party of 20 here yet?"
Server #2 "Fuck ya, and they're a bunch of fuckin greyboxes!"
Dude #1 "Hey bud, where's your car?"
Dude #2 "Some fuckin greybox ran into to me and totaled it!"