fast1 [fast, fahst] adjective, faster, fastest. 1. moving or able to move, operate, function, or take effect quickly; quick; swift; rapid: a fast horse; a fast pain reliever; a fast thinker. 2. done in comparatively little time; taking a comparatively short time: a fast race; fast work. 3. indicating a time in advance of the correct time, as of a clock. noting or according to daylight-saving time. 4. adapted to, allowing, productive of, or imparting rapid movement: a hull with fast lines; one of the fastest pitchers in baseball. 5. characterized by unrestrained conduct or lack of moral conventions, especially in sexual relations; wanton; loose: Some young people in that era were considered fast, if not downright promiscuous. 6. characterized by hectic activity: leading a fast life. 7. resistant: acid-fast.