c. 1500, "reflection of light, image produced by reflection," from a verb reflex meaning "refract, deflect" (late 14c.; compare reflect), from Late Latin reflexus "a bending back," noun use of past participle of reflectere "to bend back, bend backwards, turn away," from re- "back" (see re-) + flectere "to bend" (see flexible). Also as an adjective (1640s), "thrown or turned backward," also of thoughts or the mind. Meaning "involuntary nerve stimulation" is recorded by 1877, short for reflex action (1833) "simple, involuntary action of the nervous system."
late 14c., from Latin semen "seed of plants, animals, or men; race, inborn characteristic; posterity, progeny, offspring," figuratively "origin, essence, principle, cause," from PIE *semen- "seed," suffixed form of root *sē- "to sow."
Sense of "device that converts energy to mechanical power" is 18c.; in 19c. especially of steam engines. Middle English also had ingeny (n.) "gadget, apparatus, device," directly from Latin ingenium.
1580s, "reflective, capable of bending or turning back," from Medieval Latin reflexivus, from Late Latin reflexus (see reflect). Meaning "of the nature of a reflex" is from 1839 (implied in reflexively). Grammatical sense, in reference to verbs especially, "turning the action back upon the subject," is by 1740. Related: Reflexiveness; reflexivity.
early 15c., "intellectual, talented," from Old French ingenios, engeignos"clever, ingenious" (Modern French ingénieux), from Latin ingeniosus "of good natural capacity, full of intellect, clever, gifted with genius," from ingenium "innate qualities, ability; inborn character," literally "that which is inborn," from in- "in" (from PIE root *en "in") + gignere "to beget" (from PIE *gen(e)-yo-, suffixed form of root *gene- "give birth, beget").
Sense of "skillful, crafty, clever at contrivance" first recorded 1540s; earlier in this sense was Middle English enginous (mid-14c.), from Old French engeignos. Middle English also had engineful "skillful (in war)" (c. 1300). By a direct path, Latin ingenium produced Middle English ingeny "intellectual capacity, cleverness" (early 15c.), but this is obsolete. Compare engine. Related: Ingeniously; ingeniousness.
late 14c., "tutelary or moral spirit" who guides and governs an individual through life, from Latin genius "guardian deity or spirit which watches over each person from birth; spirit, incarnation; wit, talent;" also "prophetic skill; the male spirit of a gens," originally "generative power" (or "inborn nature"), from PIE *gen(e)-yo-, from root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups.
The sense of "characteristic disposition" of a person is from 1580s. The meaning "person of natural intelligence or talent" and that of "exalted natural mental ability, skill in the synthesis of knowledge derived from perception" are attested by 1640s.