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 4, 2007
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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
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Underwear; undergarments. Typically used as a uni-sex term referring to panties, boxers, briefs, brassieres and similar. The hyphenated spelling is grammatically correct, however alternate spellings may include: underones, under ones.
Her pants were so sheer, you could tell what color her under-ones were.
by PYLB January 11, 2008
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successfully achieved or survived
Although annoyed at the early appearance of holiday decor on the shelves, he felt it would count towards another holiday season working at the store under his belt. (example of under one's belt)
by The Return of Light Joker November 1, 2010
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Keep something a secret.

There are four origins of this idiom (I GUARANTEE IT):

- English archers protected their bows by putting the strings on their heads under their helmets;
- “keep it under our hat” was mentioned in 1982 in periodical "Gleanings in Bee Culture";
- President Abraham Lincoln's stove pipe hat, where he kept important papers;
- the ceremonial swordbearer of the Lord Mayor of London (can be dated to 1420), who keeps the key to the Lord Mayor’s seal of office in a special pocket in his hat.
-keep it under one's hat-

Joey: All right, what else?
Chandler: Well uh, there was acting classes, stage combat classes, tap classes…

Joey: Which we're still keeping under our hats!
by Kisa Boris July 13, 2016
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independently, not doing something to carry out instructions or orders
He removed the junk on his own steam. (example of under one's own steam)
by The Return of Light Joker June 26, 2009
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