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."
early 15c., "to bend inward," from Latin inflectere (past participle inflexus) "to bend in, bow, curve," figuratively, "to change, alter, influence," from in- "in" (see in- (1)) + flectere "to bend" (see flexible). Grammatical sense "to vary by change of form" (especially at the end of a word) is from 1660s. Related: Inflected; inflecting.
"bent backward," 1776, in botany, from Modern Latin retroflexus, past participle of retroflectere "to bend back," from retro "back" (see retro-) + flectere "to bend" (see flexible). The verb "to turn or fold back" (1898) is a back-formation from retroflexed (1806), which is from the adjective. Related: Retroflexion.
"act of bending the knee," especially in worship, early 15c., genu-fleccion, from Medieval Latin genuflectionem (nominative genuflexio) "bending of the knee," noun of action from past-participle stem of Late Latin genuflectere "genuflect," properly genu flectere "to bend the knee," from Latin genu "knee" (from PIE root *genu- (1) "knee; angle") + flectere "to bend" (see flexible).