1750, of sound, "unvaried in tone, characterized by monotony, unvaried in tone," from Greek monotonos "of one tone" (see monotony). Transferred and figurative use, "lacking in variety, uninteresting, tiresomely uniform," is from 1783. Related: Monotonously; monotonousness.
The secondary sense of monotonous (same or tedious) has so nearly swallowed up its primary (of one pitch or tone) that it is well worth while to remember the existence of monotonic, which has the primary sense only. [Fowler]