〈人ひと〉がくたくたになるfor a person to become completely exhausted
一いち日にち中じゅう歩あるき回まわってくたくたになった。
I walked around all day and became completely exhausted.
〈物もの〉がくたくたになるfor clothing or fabric to become worn out and shapeless
何なん年ねんも着きたセーターがくたくたになった。
The sweater I'd worn for years became all worn out.
〈物もの〉をくたくたに煮にるto boil something until it turns soft and mushy
野や菜さいをくたくたに煮に込こんだ。
I simmered the vegetables until they were soft and mushy.

Related words

ヘトヘトN2sense 1
くたくた describes general exhaustion after a long day, while へとへと emphasizes being physically drained to the point of near collapse.
ぼろぼろN3sense 2
くたくた describes clothing that's simply worn out and shapeless, while ぼろぼろ implies visible crumbling, tearing, or shedding.
ぐったりN2sense 1
くたくた suggests tiredness from exertion, whereas ぐったり often describes a limp, weakened state such as from illness or heat.
ヨレヨレ sense 2
くたくた and よれよれ both describe worn-out clothing, but よれよれ focuses more on wrinkling and loss of shape than fabric damage.
疲つかれ果はてる
くたくた is casual spoken slang for being exhausted, while 疲れ果てる is more formal or literary and works as a full verb.

Opposites

元げん気きN5sense 1
くたくた means exhausted and drained, while 元気 means energetic and full of vitality.

くたびれるN3sense 1
くたくた is the adjective describing an exhausted or worn-out state, while くたびれる is the verb meaning to become tired or worn out.

A mimetic word; the exact origin is uncertain, but the reduplicated form suggests a state of limpness or exhaustion.