monotonic adj.

word-forming element of Greek origin meaning "one, single, alone; containing one (atom, etc.)," from Greek monos "single, alone," from PIE root *men- (4) "small, isolated."

1640s, "relating to or characterized by muscular tension," from Greek tonikos "of stretching," from tonos "a stretching," from PIE root *ten- "to stretch." The meaning "maintaining the healthy firmness of tissues" is recorded from 1680s, first extended 1756 to "having the property of restoring to health." Related: Tonical (1580s).

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]