Meaning of “sink in” - Learner’s Dictionary

sink in

phrasal verb with sink us uk /sɪŋk/ verb past tense and past participle sank, also US sunk, past participle sunk

If an unpleasant or surprising fact sinks in, you gradually start to believe it and understand what effect it will have on you:

It still hasn't sunk in that I'll never see her again.

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