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.