Relating to Achilles, the legendary Greek warrior, or resembling him in character; noted for strength or martial prowess.
From Achilles (Greek Akhilleus) plus -ary (suffix meaning 'relating to'). Achilles was the central hero of Homer's Iliad, whose name has become synonymous with warrior excellence and vulnerability.
We call someone's weakness an 'Achilles heel' because Homer's Achilles had exactly one vulnerable spot—but the phrase actually misrepresents the myth, since the heel-killing is a later Roman addition to the story.
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