lung fucking is when your are having sex with a guy with a really big dick and it takes your breathe away.
(his dick is so big that measuring it up against your body, it would come up to where your lungs are.)
girl 1: "how was last night? ;D"
girl 2: "omg it was great! he was sooooo huge, i was having trouble breathing!"
girl 1: "haha you got lungfucked ;D!"