Real Dictionary

let off
phrasal verb: If someone in authority lets you off a task or duty, they give you permission not to do it.
phrasal verb: If you let off an explosive or a gun, you explode or fire it.
phrasal verb: If you let someone off, you give them a lighter punishment than they expect or no punishment at all.

let off in American English
to give forth (steam, etc.)
to release by exploding

let off in British English
to allow to disembark or leave