Etymology
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innate (adj.)
early 15c., "existing from birth," from Late Latin innatus "inborn, native, natural" (source also of French inné, Spanish and Italian innato), past participle of innasci "to be born in, originate in," from in- "in" (from PIE root *en "in") + nasci "to be born" (Old Latin gnasci), from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups. Opposed to acquired. Related: Innately; innateness.
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reflex (n.)

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."

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reflexology (n.)

1927, as a psychological theory, from German reflexologie (1912); see reflex + -ology. As a foot massage technique for releasing nervous tension, recorded by 1976.

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phototropism (n.)

"innate movement of a plant or other organism in response to the stimulation of light," 1899, based on German phototropie (1892); see photo- + tropism. Related: Phototropic.

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ingrown (adj.)
also in-grown, 1660s, "native, innate," from in + grown. Of nails, "that has grown into the flesh," 1869 (in-growing in this sense is from 1847). Related: Ingrowth (1870).
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knee-jerk (n.)
patellar reflex, a neurological phenomenon discovered and named in 1876; see knee (n.) + jerk (n.1) in the medical sense. The figurative use appeared soon after the phrase was coined.
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inborn (adj.)
Old English inboren "native to a place, indigenous, aboriginal," from in (prep.) "within" + boren "brought forth" (see born). Of personal qualities, "innate, implanted by nature," 1510s.
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reflexive (adj.)

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.

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biological (adj.)
"pertaining to the science of life," 1840, from biology + -ical. Biological clock, "innate mechanism that regulates cyclic activities of living things," is attested from 1955; not especially of human reproductive urges until c. 1991. Biological warfare is from 1946. Related: Biologically. Alternative adjective biologic is from 1850.
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engine (n.)
c. 1300, "mechanical device," especially one used in war; "manner of construction," also "skill, craft, innate ability; deceitfulness, trickery," from Old French engin "skill, wit, cleverness," also "trick, deceit, stratagem; war machine" (12c.), from Latin ingenium "innate qualities, ability; inborn character," in Late Latin "a war engine, battering ram" (Tertullian, Isidore of Seville); literally "that which is inborn," from in- "in" (from PIE root *en "in") + gignere, from PIE *gen(e)-yo-, suffixed form of root *gene- "give birth, beget."

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.
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