Old English (as a noun), from Old French adamaunt-, via Latin from Greek adamas, adamant-, 'untameable, invincible' (later used to denote the hardest metal or stone, hence diamond), from a- 'not' + daman 'to tame'. The phrase to be adamant dates from the 1930s, although adjectival use had been implied in such collocations as ‘an adamant heart’ since the 16th century.

• The Greek word adamas, originally meaning ‘invincible or untameable’, came to be applied to the hardest metal or stone and to diamond, the hardest naturally occurring substance. Via Latin it was the source not only of adamant but also of diamond. In Old English adamant was the name given to a legendary rock so hard that it was believed to be impenetrable. Early medieval Latin writers mistakenly explained the word as coming from adamare ‘take a liking to’ and associated adamant with the lodestone or magnet which ‘takes a liking’ to iron, and the word passed into modern languages with this confusion of meaning. The modern use, with its notion of unyielding conviction, is much more recent, probably dating from the 1930s.