@Hyte Enjoy a super late reply to something you don't even remember posting. Katsudon is made with pork cutlets as topping on a rice bowl, and the word thing for the katsu bit is "victory," while oyakodon is chicken and egg as the topping on a rice bowl, with oyako being "parent and child." Thus, oyakodon became slang for a threesome with a girl and her mom. I suppose you could make an argument for any variation of threesome that involves a parent and child, but mom and daughter is the use everyone's most familiar with and actually using it for.