let off
let off {v.}
1. To discharge (a gun); explode; fire.
Willie accidentally let off his father's shotgun and made a hole in the wall.
2. To permit to go or escape; excuse from a penalty, a duty, or a promise.
Two boys were caught smoking in school but the principal let them off with a warning.
Mary's mother said that she would let Mary off from drying the supper dishes.
The factory closed for a month in the summer and let the workers off.
Compare: LET GO.
3. or {informal} let off the hook
To miss a chance to defeat or score against, especially in sports or games.
We almost scored a touchdown in the first play against Tech but we let them off the hook by fumbling the ball.
The boxer let his opponent off the hook many times.
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Source: A Dictionary of American Idioms