flex (v.)

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."

also short for flexitime, 1972, translating German Gleitzeit "sliding time." See flex + time (n.).