set in phrasal verb (see also set)
if 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.

[TahlilGaran] Dictionary of Contemporary English ▲

set in
to begin.
If the wound is not treated, infection may set in.

In winter, darkness sets in so early!

[TahlilGaran] English Idioms Dictionary ▲

set in
to begin and probably continue (used for a weather or mental condition)
The rain has set in and it looks like it will not stop for awhile.

A mild depression has set in for my neighbor.

[TahlilGaran] English Idioms Dictionary ▲

set in
v. To begin; start; develop.
Before the boat could reach shore, a storm had set in.
He did not keep the cut clean and infection set in.
The wind set in from the east.

[TahlilGaran] English Idioms Dictionary ▲