What does "act up" mean?

For a body part to cause problems For a part of the body to malfunction or cause pain.

How do you use "act up" in a sentence?

My back is acting up again. His knee acts up in cold weather. My stomach has been acting up.

Phrasal Verb BritishAmericanAustralian ★★★★☆ Common informal

Meaning: For a body part to cause problems

（身体部位）出毛病

Dar problemas, molestar

（体の一部が）調子が悪くなる

(신체 부위가) 말을 안 듣다

For a part of the body to malfunction or cause pain.

身体某部分不能正常工作或引起疼痛。常用于描述旧病或老伤复发的情况。

Para una parte del cuerpo que no funciona bien o causa dolor. Se usa a menudo para describir el empeoramiento de una dolencia antigua o una lesión.

体の一部がうまく機能しなくなったり、痛みを引き起こしたりすること。持病や古傷について使われることが多い。

신체 일부가 제대로 기능하지 않거나 통증을 일으키다.

Base verb: act

Particle: up

Separable: No — the verb and particle stay together

My back is acting up again.
我的腰又出毛病了
Mi espalda me está dando problemas otra vez
また腰の調子が悪い
또 허리가 말을 안 듣는다.
His knee acts up in cold weather.
他的膝盖一到冷天就不舒服
Su rodilla le molesta cuando hace frío
彼の膝は寒いと調子が悪くなる
그의 무릎은 추우면 말을 안 듣는다.
My stomach has been acting up.
我的胃一直不舒服
El estómago me ha estado dando guerra
胃の調子がずっと悪い
위장이 계속 안 좋다.

Context: pain, malfunction, chronic

Tone: frustrated

From acting badly.

Era: Modern

Generation: All ages

Social background: Universal