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.

the thing that you don’t have when you’re phones percentage is over 20

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.