A person who gives up easily or cannot stick with something for long; someone who quits after a very short time.
ジョギングを始めたけど、三日坊主で続かなかった。
I started jogging, but I was a three-day monk and couldn't keep it up.
毎年ダイエットに挑戦するが、三日坊主で終わることが多い。
I try dieting every year, but often give up after a few days.
彼は飽きっぽくて三日坊主だと言われている。
He's said to be fickle and a quitter.
三日坊主にならないように、スケジュールを小さく区切って始めた。
To avoid giving up after a few days, I started by breaking the schedule into small tasks.