Definition of "set in"

set in

verb
third-person singular simple present sets in, present participle setting in, simple past and past participle set in
1. To take root, become established.
  Quotations
  [...] but rain set in, driven by a furious westerly gale, [...].
  1941 November, Cecil J. Allen, “British Locomotive Practice and Performance”, in Railway Magazine, page 484
  By the afternoon it seemed as if the storm had passed and that frost was setting in; but in the evening the wind rose to gale force, bringing telegraph poles down like skittles and tangling power and telephone lines.
  1960 March, “The January blizzard in the North-East of Scotland”, in Trains Illustrated, page 137
  Work on anti-frost precautions on diesel locomotives is to be speeded up to ensure that most if not all locomotives have been dealt with before the winter sets in.
  1964 September, “Motive Power Miscellany: BR Workshops”, in Modern Railways, page 220