From Longman Dictionary of Contemporary Englishset in phrasal verbSTART TO HAPPEN, EXIST ETCif something sets in, especially something unpleasant, it begins and seems likely to continue for a long time Winter seems to be setting in early this year. Further economic decline set in during the 1930s. → set→ See Verb table
Examples from the Corpus
set in• The doctors operated immediately to prevent any infection setting in.• Worldwide economic recession set in during the early 1980s.• It looks as if the rain has set in for the day. 