One-upmanship is the art or practice of successively outdoing a competitor by discomfiting him.
The term was first used in the title of a book by Stephen Potter, published in 1952 as a follow-up to The Theory and Practice of Gamesmanship (or the Art of Winning Games without Actually Cheating) (1947), and Lifemanship titles in his series of tongue-in-cheek self-help books, and film and television derivatives, that teach various ploys to achieve this. This satire of self-help style guides manipulates traditional British conventions for the gamester, all life being a game, who understands that if you're not one-up, you're one-down. Potter's unprincipled principles apply to almost any possession, experience or situation, deriving maximum undeserved rewards and discomforting the opposition. The 1960 film School for Scoundrels and the 2006 film School For Scoundrels were satiric portrayals of how to use Potter's ideas.
In that context, the term refers to a satiric course in the gambits required for the systematic and conscious practice of "creative intimidation", making one's associates feel inferior and thereby gaining the status of being "one-up" on them. Viewed seriously, it is a phenomenon of group dynamics that can have significant effects in the management field: for instance, manifesting in office politics.
Also known as a: One Upper, One-upper, being Shaidled.