A huge erection. Not at half staff, this kind of boner is in full salute. If up for too long you get light headed.. The kind that garners positive or shocked reactions.
"Daym look at yr boneskirt, that thing is a monster!"
Otherwise known as a side chick, backup girl, second choice sally, who you call for assistance when your main is busy.
"Alyssa will always be a backburner bitch, she doesn't have what it takes to be anyones number one girl"