weak-willed; lacking the willpower or resolve to persevere, resist impulses, or follow through—expressed as a four-character idiom (yojijukugo).
彼は意志薄弱でダイエットが続かない。
He is weak-willed and can't stick to a diet.
注意されているのに改善せず、周りから意志薄弱だと言われた。
Even though he was warned, he didn't improve and people called him weak-willed.