Besides being a delicous type of Japanese food, oyako donburi is also a japanese term meaning for a person to have a physical sexual relationship with both a mother and her daughter. This does not necessarily mean at the same time and have a 3p.

This is not necessarily a slang meaning in japanese, since it is found in the dictionary and DOES NOT mean incest, that is called kinshin soukan by the way and is kimochi warui. Unfortunately, I don't know the etymology or time period from which this interesting meaning came.

If you don't believe me, refer to below online dictionaries. Like they are really gonna print that kind of stuff in Japanese to English dictionaries for sale outside of Japan.
haha oya to sono musume (or can be read as "ko") to no ryouhou to nikutai kankei wo motsu koto.