Meaning of “let sb off” - Learner’s Dictionary

let sb off

phrasal verb with let us uk /let/ verb [ T ] present participle letting, past tense and past participle let

B2 to not punish someone who has done something wrong, or to not punish them severely:

I'll let you off this time, but don't ever lie to me again.
The judge let her off with (= only punished her with) a fine.

(Definition of “let sb off” from the Cambridge Learner’s Dictionary © Cambridge University Press)