What is a adverb for monoutonous?

There isn't a single perfect adverb for "monotonous". However, depending on the context you can use these options:

To describe something that is dull and boring:

* Dully: "The lecture droned on dully."

* Tediously: "They spent the afternoon tediously sorting through old papers."

* Monotonously: "The rain fell monotonously for days."

* Repetitively: "The music played repetitively in the background."

* Wearisomely: "The journey was wearisomely long."

To describe something that is lacking in variation:

* Uniformly: "The landscape was uniformly flat."

* Consistently: "The machine ran consistently, without any problems."

* Regularly: "The clock ticked regularly."

* Evenly: "The paint was evenly spread."

The best choice will depend on the specific meaning you want to convey.