grizzled hair; hair streaked with gray
Describes hair that is mixed with gray or white, typically from aging. The phrase is a fixed compound; 白髪交じり can also be used attributively.
My father keeps his grizzled hair cut short.
His hair streaked with gray makes him look his age.
Compound of 白髪 (しらが, 'gray hair') + 交じり (まじり, 'mixed with') + の + 髪 (かみ, 'hair'). The exact historical derivation is straightforward; the phrase is a transparent combination of its parts.