monotony (n.)

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.