monoton
monotonous
Uniform and without variety, causing something to be perceived as boring or tiring.
Die Arbeit am Fließband ist sehr monoton.
Work on the assembly line is very monotonous.

monotonous / monotone
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monotonous
Uniform and without variety, causing something to be perceived as boring or tiring.
Die Arbeit am Fließband ist sehr monoton.
Work on the assembly line is very monotonous.

The word comes from the Greek 'monos' (single, alone) and 'tonos' (tone). This origin explains both meanings: an activity with only one 'tone' (without variety) is boring, and a voice with only one 'tone' sounds uniform.
Das monotone Geräusch des Motors machte ihn schläfrig.
The monotonous sound of the engine made him sleepy.
The context makes clear which meaning is intended. If 'monoton' (monotonous / monotone) refers to an activity, a process, or a landscape, it means 'boring'. If it refers to a voice or a sound, it means 'uniform in tone'.
Sein Alltag war monoton und ohne Überraschungen.
His everyday life was monotonous and without surprises.
In the meaning of 'langweilig' (boring), 'monoton' (monotonous) is often used to describe repetitive activities or an uneventful daily life. Typical phrases are 'monotone Arbeit' (monotonous work), 'monotone Bewegungen' (monotonous movements), or 'ein monotones Leben' (a monotonous life).
Nach stundenlanger monotoner Fahrt auf der Autobahn brauchte sie eine Pause.
After hours of monotonous driving on the highway, she needed a break.
boring
'langweilig' and 'monoton' are close in meaning when something offers no variety. 'langweilig' mainly describes the effect on a person: you are not interested and feel bored. 'monoton' focuses more on the form: something stays the same, repeats itself, or sounds without variation. That is why 'monoton' fits well with work, noises, voices, or processes. Not everything boring is monotonous, but monotony can make something boring.

Die Stimme des Lehrers klang monoton, und viele wurden müde.
The teacher's voice sounded monotonous, and many people got tired.
Nach zehn Minuten wurde das Spiel für die Zuschauer langweilig.
After ten minutes, the game became boring for the spectators.