When you're at a house party, and you've drinken too much, and go to lay down in an empty room and inadvertently end up in bed with 5 or more people. While everyone else either falls off or leaves, one, or even two couples will stay and makeout on you. Literally on you, making you their 'mattress'.
Person1: Hey baby, last night was great! ;)
Person2: Yeah it was! But your bed felt kind of weird.
Person1: Oh yeah!! It was because Paris was mattressing for us!