early 15c., "capable of being bent; mentally or spiritually pliant," from Old French flexible or directly from Latin flexibilis "that may be bent, pliant, flexible, yielding;" figuratively "tractable, inconstant," from flex-, past participle stem of flectere "to bend," which is of uncertain origin. Flexile (1630s) and flexive (1620s) have become rare. Related: Flexibly. Coles' dictionary (1717) has flexiloquent "speaking words of doubtful or double meaning."
c. 1600, "bent part," also, in grammar, "modification of part of a word," from Latin flexionem (nominative flexio) "a bending, swaying; bend, turn, curve," noun of action from past participle stem of flectere "to bend" (see flexible). Flection (18c.) is more recent, less etymological, but said to be more common in modern English, perhaps by influence of affection, direction, where the -ct- is in the Latin word. According to some modern dictionaries, flexion is "confined to anatomical contexts." Related: Flexional; flectional.
<a href="https://www.etymonline.com/word/flexor">Etymology of flexor by etymonline</a>
Harper, D. (n.d.). Etymology of flexor. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/flexor