Green’s Dictionary of Slang

lilac n.

[the supposed similarity of a bushy sideburn to a lilac flower]

(US) in pl., sideburns or sideboards.

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[US]C. Sandburg ‘Alley Rats’ in Smoke and Steel 20: They were calling certain styles of whiskers by the name of ‘lilacs.’ / And another manner of beard assumed in their chatter a verbal guise / Of ‘mutton chops,’ ‘galways,’ ‘feather dusters’.