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monotony; monotone; dullness
monotone; monotonic
毎日同じことの繰り返しで単調な生活に飽きてきた。
I'm getting tired of the monotonous life of repeating the same things every day.
単調なメロディだが心に響く何かがある曲だと思う。
It's a monotonous melody, but I think it has something that resonates.
単調な作業が続くと集中力が低下してしまいがちだ。
Concentration tends to decline when monotonous work continues.