The full phrase is an example of or "trash talk." It is designed to be nonsensical yet highly provocative by combining a sexual act with a family member and a specific number to make the "claim" sound like a recorded statistic.
: Some sources suggest an alternative origin in prison culture, where it was allegedly used to describe a forced act. Non-Sexual Slang your mom tossed my salad 21 top
, which use the phrase in its literal culinary sense for comedic effect. What Is Tossing Someones Salad? Sex Slang Explained The full phrase is an example of or "trash talk