noun
quitter; someone who can't stick to anything
Describes a person who gives up easily or never finishes what they start. Literally 'three-day monk', from the idea of someone who enters a monastery but leaves after only three days.
彼は何をやっても三日坊主だ。
He never sticks with anything; he's a quitter.
ダイエットはいつも
My diets always fizzle out after a few days.