1...someone who continually shits their pants to the point they just wear none, and only wear diapers.
2...a person who goes insane, and the only thing they can think of doing at the time is shit themselves, therefore ruining thier pants, and underwear.
3...a vandal who goes into expensive stores and tries on all of the white pants, and leaves big brown stains in all the asses, in effect ruining them.
4...when in battle, shitting in your BDU's and calling for a medic to come and change them for you so you can keep fighting on, and killing people, not wasting time worrying about clean pants, that's the Medic's job.
Blap is such a Pants Ruiner, he dosn't even come out of the house anymore, and just lines his walls with plastic.
Wang lost his wife and kids to a stampede of Yetti, so when he found out he was a true Pants Ruiner due to the shock of losing his whole family.
Let's go out Pants Ruining tonight, and show those motherfukers who is the real boss.
Specialist Chris Shupe is the greatest Pants Ruiner in the Army, he killed twelve Arabs all while getting his pants changed, what a bad-ass motherfuker.