godan verb (-mu), intransitive verb
become lukewarm; become tepid
Describes something that was hot or cold reaching a lukewarm temperature, often with a nuance of losing desirable heat or coolness.
お風呂のお湯がぬるんでしまった。
The bath water has become lukewarm.
冷蔵庫から出したビールがすぐにぬるんだ。
The beer I took out of the fridge quickly became tepid.