1A shortage or insufficiency; the state of not having enough of what is needed (money, time, sleep, manpower, ability, etc.). Very common in compounds like 睡眠不足 (lack of sleep) and 人手不足 (labor shortage).
睡眠不足で一日中頭がぼーっとしていた。
I was in a daze all day from lack of sleep.
人手不足で、店を早めに閉めることにした。
We decided to close the shop early because we're short-staffed.
運動不足を解消するために、毎朝走り始めた。
I started running every morning to make up for my lack of exercise.
経験が不足しているので、もっと現場で学ぶ必要がある。
I lack experience, so I need to learn more on the job.
資金が不足していて、計画を一部見直さざるを得なかった。
We were short on funds and had to revise part of the plan.
2Dissatisfaction or discontent; a feeling that something falls short of what one wants, as in 不足を言う (to grumble) or 不足はない (to have no complaints). Somewhat formal and old-fashioned.
これだけの待遇で、まだ不足だと言うのか。
You're getting all this and you're still not satisfied?
彼は今の暮らしに何の不足も感じていない。
He feels no dissatisfaction whatsoever with his current life.