When a corncob is used to improvise a sexual act, when you do not have/or have used up any sexual toys eg dildo. Naturally during a sex scene in a pornographic movie.
Person #1: Got the dildo?
Person #2: No, we're gonna have to use a Porn on the Cob
Person #1: Aww, okay...
While having anal sex with a woman, you pull out to discover there is a piece of undigested corn on your dick.
I was at a party on the week-end and this cougar totally wanted my dick in her ass. About half way through I pulled out to change positions and realized we were totally putting the corn back on the cob. In the interest of fairness I asked her eat the leftovers. That was awesome!
When the penis is too big to fit inbetween someone's asscheeks or pussy flaps, so they have to sit on top of it like they would a horse, and slide up and down.
Joel: What the fuck are you doing, Courtney?!
Courtney: I'm corning on the cob, Joel!