It's when a girl (or a guy) is riding you with her front facing you and you start poopin in your hands secretly. Then you take the poop and you rub it up and down the girl's back like a washboard.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed

Actually, it is not necessary to "snot rocket" down the washboardee's throat, but rather they may clean out the washboarder's nostrils with their tongue. This alternate strategy is also deemed a washboard.