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1. A shrewd, tough person.
2. A stubborn, unmovable person.
3. pl. hardhead or hard·heads Any of several fishes having a bony head, especially the Atlantic croaker.
(Plants) (functioning as singular) a thistle-like plant, Centaurea nigra, native to Europe and introduced into North America and New Zealand, that has reddish-purple flower heads: family Asteraceae (composites). Also called: knapweed See also centaury2