Noun: A sexual act typically performed during three way sex where the female is receiving vaginal or anal sex on all fours (doggy style) while the third party lies underneath and licks or otherwise performs oral sex on the female and possibly the male concurrent with their intercourse. The third party may be either female or male.
Verb: To perform oral sex on a female and possibly also the male while they are having intercourse (fucking).
Bro, I was banging Kristen last night and Linda underdogged us. It felt like getting pussy and head at the same time.