To loosen, sag, or droop — losing tension or tautness (physical sagging or slackening).
このベルトは長年使っていてたるんでいる。
This belt has become slack after years of use.
屋根の梁がたるんでいるので修理が必要だ。
The roof beam is sagging, so it needs repair.
シーツがたるんで寝心地が悪い。
The sheets are loose and it's uncomfortable to sleep.
To slack off in effort or attention; to become lax or let one's guard down (in work or behavior).
試験が終わって気がたるんでいる学生が多い。
Many students have let their guard down after the exam.
最近、業務の進み方がたるんでいるので注意した方がいい。
Work progress has been slack recently; it's better to pay attention to that.
準備を怠ると後でたるみが出るから、今からしっかりやろう。
If you neglect preparation you'll fall behind later, so let's do it properly from now.