Having grey or grey-streaked hair, especially from age; looking old and weathered, or worn by time and experience.
From 'grizzle' (to make grey) plus past participle '-ed'; describes someone who has been greying from time and experience.
A 'grizzled veteran' sounds tough and experienced because grey hair literally signals someone who has survived enough to age—Hollywood uses this look to show competence and toughness.
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