無気力（むきりょく） means lethargic, apathetic, or lacking motivation. It combines 無（む） “no / absence” with 気力（きりょく） “energy, willpower.”
Use it when someone has little mental drive to act, care, or engage. The focus is usually mental or emotional energy, not simple physical tiredness.
It can describe a short-term slump, but it is also often used around burnout, as in 燃え尽き症候群（もえつきしょうこうぐん）. It may also appear when talking about depression, so avoid using it too lightly for someone else’s serious condition.
Grammar note: 無気力 works as both a noun and a な-adjective, as in 無気力な状態（むきりょくなじょうたい） “an apathetic state.”