A dirty sock condom is the condom that has been in the bottom of your sock drawer since your dad gave it to you during your freshman year of high school.
Same as a normal sock surprise, however, it is done with a used sock and going commando. The resulting odor can be used as either an aphrodisiac or as a way to disorientate your new friend.
guy 1: Did you pull off that sock surprise last night?
guy 2: You bet!...but I did a dirty sock surprise. As soon as I pulled that magic out and she caught a whiff, it was as if candles lit the room and kenny g played played a single breath solo...she didn't care about the size of my penis anymore, love was in the air!