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]
"study of punishment for crime and crime prevention," 1838, coined apparently by Francis Lieber, corresponding member of the Philadephia Society for Alleviating the Miseries of Public Prisons, from pen- as in penitentiary (ultimately from Latin poena "penalty, punishment;" see penal) + -ology "study of." Related: Penologist; penological.
1721, "loss of feeling," medical Latin, from Greek anaisthēsia "want of feeling or perception, lack of sensation (to pleasure or pain)," abstract noun from an- "without" (see an- (1)) + aisthēsis "feeling," from PIE root *au- "to perceive." For the abstract noun ending, see -ia. As "a procedure for the prevention of pain in surgical operations," from 1846. Aesthesia "capacity for feeling" is attested in English from 1853, perhaps a back-formation.
1670s, in botany, "rootlet, part of the embryo of a plant which develops into the primary root," from Latin radicula, diminutive of radix "root" (from PIE root *wrād- "branch, root"). Anatomical sense of "branch of a nerve, vein, etc. resembling a root" is by 1830.