monotonous (adj.)

1706, originally in transferred sense of "wearisome sameness, tiresome uniformity or lack of variation," from French monotonie (1670s), from Greek monotonia "sameness of tone, monotony," from monotonos "of one and the same tone," from monos "single, alone" (from PIE root *men- (4) "small, isolated") + tonos "tone," from PIE root *ten- "to stretch." Literal sense of "sameness of tone or pitch" is attested in English from 1724.

in music, etc., "of or pertaining to a single, unvarying note," 1797; see mono- + tonic (adj.). Related: Monotonically.

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, 1926]