Etymology
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die-hard (n.)

also diehard, 1844, in reference to the 57th Regiment of Foot in the British Army, from the verbal phrase die hard "suffer, struggle, or resist in dying," 1784; see die (v.) + hard (adv.). As an adjective, attested from 1871. The  brand of automobile battery, spelled DieHard, was introduced by Sears in 1967.

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Definitions of die-hard

die-hard (adj.)
tradition-bound and obstinately opinionated;
an inflexible (or die-hard) conservative
Synonyms: rock-ribbed
From wordnet.princeton.edu