In a manner that is tedious, repetitious or lacking in variety.(adverb)
In a droning manner, that does not change pitch.(adverb)
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Use "monotonously" in a sentence
"Each morning, when zoo dawn arrived, a starling gushed a medley of stolen songs, distant wrens cranked up a few arpeggios, and cuckoos called monotonously like clocks stuck on the hour."
"These speeches over, the roll-call monotonously continued, each State voting as before until Wisconsin changed from"
"She was a dainty little widow, with a manner which she supposed to be pleasant and ingratiating but which the girls termed monotonously servile."