It is a noun in Japanese language that is derived from “mannerism“.
It means “to be stuck in a rut“. It is used to describe situations in which the way of something is stereotyped or it is repeatedly done.
“マンネリ化する“ is often used as the verb form.
It’s been 6 months since Ken and Leo started self-quarantining because of the COVID-19 pandemic. They are having a video call.
- Ken: Hi. How are you doing?
元気？ - Leo: I'm good,but it’s too boring to stay home every day. What do you do to kill time at home?
うん。でも、毎日家に居るのは退屈過ぎる。どうやって暇つぶしているの？ - Ken: I read books, watch Netflix and play video games. My life has become a routine.
本(を)読んで、ネットフリックス(を)見て、ゲームしてる。マンネリ化している。 - Leo: I know how you feel. But I don’t know how to get out of this rut.
わかる。でも、このマンネリから抜け出す方法が分からないよ。
Meg is talking about her date with her boyfriend.
- Amy: What did you do with Tom last weekend?
先週末は、トムと(のデートは)どうだった？ - Meg: We worked out at the gym in the morning, went shopping and had dinner at a neighbourhood Italian restaurant. Our dates are stuck in a rut.
午前中ジムに行って、買い物をして、近所のイタリアンで夜ごはんを食べたよ。デートがマンネリ化しちゃっている。 - Amy: Why don’t you go somewhere new for a change?
気分転換に、どこか新しい所へ行ってみたら？ - Meg: Yes. Actually, we’re planning to climb a mountain before it is cold.
うん。実は、寒くなる前に、山登りに行く予定なんだ。