Meaning of “set in” - Learner’s Dictionary

set in

phrasal verb with set uk us /set/ verb (present participle setting, past tense and past participle set)

If something unpleasant sets in, it begins and seems likely to continue:

This rain looks as if it has set in for the rest of the day.

(Definition of “set in” from the Cambridge Learner’s Dictionary © Cambridge University Press)