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1.
A tactic to silence a One upper.
The situational One under:
Adam and his girlfriend are sitting down to dinner at the local pub accompanied by a notorious "One upper", the "One upper" notices Adam's girlfriend is carrying around a Kate Spade. He says, "Is that a Kate Spade?", asking this assuming that it is and wanting to make up a story that he bought his girlfriend, or his "one" girlfriend ("One uppers" often claim multiple hot girlfriends depending on who they are around) 2 Kate Spades and probably a real Gucci bag or two...who the fuck knows how many...anyway, instead of "Yes, it is a Kate Spade." She replies, "No, it's a fake one."
This response stumps the "One upper". How does one "one up" that? He's been "one-undered"!
by Buffalo Kelly February 18, 2007
 
2.
A term usually used amongst London Underground staff, a one under is a person who has either intentionally or accidentally become trapped underneath a tube train. This action causes large delays.

Most one unders are suicide attempts, but occasionally accidental. A lot of one unders are fatal, but the 'luckier' people are just badly mangled by the sharp bits underneath the train.

There are anti-suicide measures put in place on many London Underground lines including deep trenches and glass barriers. The trenches are so that people can't easily lie on the tracks and people who accidentally fall can protect themselves by lying in the trenches, and the glass barrier doors only open when a train is on the platform.
We have another one under at Stepney Green station.
by Okonius October 04, 2007