noun
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.
父は白髪交じりの髪を短く刈っている。
My father keeps his grizzled hair cut short.
白髪交じりの髪が年齢を感じさせる。
His hair streaked with gray makes him look his age.
白髪交じり is a noun or no-adjective meaning 'streaked with gray,' often used attributively (e.g., 白髪交じりの男性). 白髪交じりの髪 is the full phrase for 'grizzled hair.'
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.