In a firm, unyielding, and determined way that is impossible to change.
From Middle English 'adamaunt,' from Old French 'adamant,' from Latin 'adamantem' (unconquerable), from Greek 'adamas' (hardest steel/diamond). The sense evolved from literal hardness to metaphorical stubbornness.
The word 'adamant' comes from ancient beliefs that diamond was literally the hardest substance in the universe—so unbreakable that medieval people thought it had magical properties! Saying something 'adamantly' means you're as immovable as diamond.
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