How to Say "Moody" in Japanese

気分屋

kibun'ya[kʲibɯ̟̃ɰ̃ja̠]

The Japanese word for "moody" is 気分屋 (kibun'ya).

気分屋 is used to describe someone whose personality or temperament is characterized by unpredictable changes in mood.

Meaning: Given to sudden or frequent changes of mind, temperamental (adj)

How does it change in a sentence?

Because 気分屋 is a noun acting as a na-adjective, you use 気分屋な to modify a noun directly, such as 気分屋な人 (a moody person).

The suffix 屋 (ya) in 気分屋 originally referred to a shopkeeper or a person who specializes in a certain trade, such as a 'fish seller'. In this context, it implies the person is a 'specialist' in their own moods.
If someone is acting moody in the moment, Japanese speakers use 気分が乗らない (kibun ga noranai), which literally means their mood is 'not riding' or 'not boarding'.
The character 気 (ki) in 気分屋 refers to the concept of 'life energy' or 'spirit' that flows through the body, suggesting that moodiness is a fluctuation of internal energy.

気分屋なんだから。
You are moody.
Tatoeba #11571947 by small_snow, translation #9631758 by megamanenm
トムって気分屋だよね。
I think Tom is moody.
Tatoeba #3594833 by arnab, translation #3183887 by CK
あの人って本当に気分屋ね。私、ついていけない。
He's so moody I have trouble keeping up with him.
Tatoeba #230784 by KK_kaku_, translation #68151 by CK
結婚式前の彼女は気難しくなりました。
She became moody before the wedding.
最近、彼は不機嫌な様子です。
Lately, he has been in a bad mood.
今日はなんだか変な気分です。
I'm feeling a bit moody today.
2 more sentences on Tatoeba

Yui and Kenta are talking about their boss at work.

結衣 Yui

部長は本当に気分屋だね。
Buchō wa hontō ni kibun-ya da ne.
The manager is really moody, isn't he?
健太 Kenta

そうだね。機嫌に波があるよ。
Sō da ne. Kigen ni nami ga aru yo.
That's true. His mood fluctuates a lot.
結衣 Yui

接し方が難しいなあ。
Sesshikata ga muzukashii nā.
It's hard to know how to deal with him.
健太 Kenta

僕も困っているよ。
Boku mo komatte iru yo.
I'm struggling with it too.

Sources

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