let offphrasal verb
Definition of let off
1 : to allow (someone) to get off a bus, an airplane, etc. Could you let me off (the bus) at the next stop, please? The bus stopped to let off a few passengers.
2 : to allow (someone who has been caught doing something wrong or illegal) to go without being punished The police officer let her off with just a warning. They let him off easy/easily/lightly, if you ask me.
3 : to cause (something) to explode or to be released in a forceful way let off a firecracker opened the valve to let off pressure