Lock

Trader slang for a position with a guaranteed profit regardless of the outcome, on prediction markets, typically buying every outcome of a one-winner event whose field costs under 100 cents (an underround).

True locks are the unicorns of trading: real when they appear, gone in minutes, and smaller than they look once fees and fill prices are counted. The honest use of the word is narrow, a structural mispricing you verified leg by leg, not a bet you merely feel great about, which is how the word usually gets people in trouble.