This is the 7544th most frequent Japanese word.
冷え込ん
"冷え込ん" means "to become very cold" or "to chill significantly."
The verb 冷え込む here describes the significant drop in temperature.
昨夜は冷え込んで、とても寒かったです。
Last night, it got very cold, and it was freezing.
冷え込む is used to indicate the distinctive cool weather pattern during seasonal changes.
秋になると、朝晩が冷え込むことがよくあります。
As autumn arrives, mornings and evenings often get chilly.
冷え込む is presented in its future use to inform about the impending temperature drop.