Originally used late one night on AIM back when that was usable by 'Spud The Destroyer', and adapted as a response from "Okay". yep is to yes, as kape is to okay.
Earliest chatlog backups indicate its usage as early as late 1999.
Girl 2: OMG! What happened? Was it any good?
Girl 2: It was disgusting! I wanted a nice soft, romantic kiss and he 'kaped' me! I felt so violated I needed to brush my teeth 15 times, then have four showers.
Girl 2: Awww, you poor thing! I hope you didn't give him your number!
Girl 2: No way! I'm going to write an online blog about him, to warn other girls. Yuck!