A squaddy mattress is a girl who's slept with more than 5 members of our lovely armed forces. General slag, scally and chav.
"That Laura is a right squaddy mattress she's shagged 17 of them!"
A Lady, Woman or Girl that thinks it's a turn on to fuck all of the british army whilst on a night out. The Squaddy Mattress earn their keep by getting free drinks from the from the unsuspecting soldier in return for back ally action.
"Lets get out of here this place is full of Squaddy Mattress's"