1630s, "that cannot be tamed or subdued," from Late Latin indomitabilis "untameable," from in- "not, opposite of, without" (see in- (1)) + *domitabilis, from Latin domitare, frequentative of domare "to tame" (see tame (adj.)). In reference to persons or personal qualities, "unyielding, persistent, resolute," by 1830. Related: Indomitably.
mid-13c. (late 12c. as a surname), of animals, "ferocious;" c. 1300, "wild, undomesticated, untamed," also "wild, uncultivated" (of land or places), from Old French sauvage, salvage "wild, savage, untamed, strange, pagan," from Late Latin salvaticus, alteration (vowel assimilation) of silvaticus "wild, woodland," literally "of the woods," from silva "forest, grove" (see sylvan).
Of persons, "indomitable, valiant," also "fierce, bold, cruel" (c. 1300); from late 14c., of persons or behavior, "wild, barbarous, uncivilized;" c. 1400 as "reckless, ungovernable," and by 1610s as "pertaining to or characteristic of savage peoples, living in the lowest condition of development." In heraldry, "naked or clothed in foliage" (1570s). The -l- often was restored in 16c.-17c. English spelling.
"a very hard stone," mid-14c., adamant, adamaunt, from Old French adamant "diamond; magnet" or directly from Latin adamantem (nominative adamas) "adamant, hardest iron, steel," also used figuratively, of character, from Greek adamas (genitive adamantos), the name of a hypothetical hardest material.
It is a noun use of an adjective meaning "unbreakable, inflexible," which was metaphoric of anything unalterable (such as Hades) and is of uncertain origin. It is perhaps literally "invincible, indomitable," from a- "not" (see a- (3)) + daman "to conquer, to tame," from PIE root *deme- "to constrain, force, break (horses)," for which see tame (adj.). "But semantically, the etymology is rather strange," according to Beekes, who suggests it might be a foreign word altered in Greek by folk etymology, and compares Akkadian (Semitic) adamu.
Applied in antiquity to a metal resembling gold (Plato), white sapphire (Pliny), magnet (Ovid, perhaps through confusion with Latin adamare "to love passionately"), steel, emery stone, and especially diamond, which is a variant of this word. "The name has thus always been of indefinite and fluctuating sense" [Century Dictionary]. The word had been in Old English as aðamans, but the modern word is a re-borrowing.