A term used to acknowledge the utter defeat and mastery of a woman's vagina by its opposing penis. Similar to the "knockout" used in the beautiful sport of MMA, victory in the war-zone that is a bed (or anywhere else you choose to do the deed) can be obtained by mutual submission (aka: exhaustion) or by a PKO.
Characteristics of PKO include, but are not limited to: female's inability to take in more thrusts; massive vaginal cum; female loses consciousnesses; muscle spasms in vaginal area and upper thighs, etc...
"Wanna go home and get PKO'd, Lauren?"
"Don't ask, Carrie... after last night's PKO I will need minimum two week recovery time."