retroflex (adj.)

word-forming element of Latin origin meaning "backwards; behind," from Latin retro (prep.) "backward, back, behind," usually in reference to place or position, rarely of time, "formerly, in the past," probably originally the ablative form of *reteros, based on re- "back" (see re-).

L. retro stands to re- as intro, "in, within"; to in, "in," and as citro, "hither," stands to cis, "on this side." [Klein]

Common in combinations in post-classical Latin (the classical equivalent was post-). Active in English as a word-forming element from mid-20c.

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."