Reflex
early 16th century (as a noun denoting reflection): from Latin reflexus ‘a bending back’, from reflectere ‘bend back’ (see reflect).
wiktionary
From Late Latin reflexus, past participle of reflectere(“to bend back”). Photography sense is from noun sense meaning “reflection”.
etymonline
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."
