A logical fallacy sometimes used in debates, wherein a party feels that their argument has been refuted, and attempts to back out of their own argument by claiming to have been trolling all along. This is invalid, for a successful troll would never have any reason to admit to trolling, as letting the trolled know that they have been trolled defeats the purpose of trolling in the first place.
Ad trollinem can occur IRL, and can even be used by those unfamiliar with the act of "trolling". In these cases, the party claims that they were kidding or weren't serious about their argument in the first place, in an attempt to disassociate their selves with it.
Party A: Hey why is the sky blue?
Party B: The sky is blue because the ocean is blue and what you're looking at is a reflection of the ocean.
Party C: Actually, the molecules of gases in the atmosphere scatter light of short wavelengths, so the white light that enters the atmosphere is perceived as blue light from the ground.
Party B: Relax dude, I was just trolling.
Party C: Bullshit. I'm calling Ad Trollinem on that.