late 14c., reflecten, "turn or bend (something) back, reverse;" early 15c., "to divert, to turn (something) aside, deflect," from Old French reflecter (14c.), from Latin reflectere "to bend back, bend backwards, turn away," from re- "back" (see re-) + flectere "to bend" (see flexible). Of mirrors or other bodies or surfaces, "to shine back light rays or images," early 15c., later also of heat or sound. The meaning "to turn one's thoughts back on, resolve matters in the mind" is from c. 1600. Related: Reflected; reflecting.
Middle English also had a separate verb reflexen "refract (light); deflect" (early 15c.), directly from Latin reflexus, past participle of reflectere.