late 14c., "incapable of being bent, physically rigid," also figuratively, "unyielding in temper or purpose," from Old French inflexible and directly from Latin inflexibilis "that cannot be bent," from in- "not, opposite of" (see in- (1)) + flexibilis "pliant, yielding" (see flexible). In early 15c. an identical word had an opposite sense, "capable of being swayed or moved," from the other in- "in, on" (see in- (2)). Related: Inflexibly.
late 14c., obstinacie, "hardness of heart, inflexibility of temper or purpose," from Medieval Latin obstinatia, from obstinatus "resolute, inflexible, stubborn" (see obstinate).
in musical phrases, "recurring frequently, repeated," 1876, from Italian ostinato "obstinate, persistent," from Latin obstinatus "resolute, resolved, determined, inflexible, stubborn," past participle of obstinare "to persist" (see obstinate).
Old English stif "rigid, inflexible," from Proto-Germanic *stifaz "inflexible" (source also of Dutch stijf, Old High German stif, German steif "stiff;" Old Norse stifla "choke"), from PIE *stipos-, from root *steip- "press together, pack, cram" (source also of Sanskrit styayate "coagulates," stima "slow;" Greek stia, stion "small stone," steibo "press together;" Latin stipare "pack down, press," stipes "post, tree trunk;" Lithuanian stipti "to stiffen, grow rigid," stiprus "strong;" Old Church Slavonic stena "wall"). Of battles and competitions, from mid-13c.; of liquor, from 1813. To keep a stiff upper lip is attested from 1815. Related: Stiffly.
name of one of the three judges of the Underworld in Greek mythology, son of Zeus and Europa, from Latinized form of Greek Rhadamanthos, whose name seems to contain Greek rhadamos "branch, twig, shoot." Used in English from 1580s allusively of inflexible judges or solid and final judgment. Related: Rhadamantine; Rhadamanthean.