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.”
Metaphors such as these sprang from their lips while other street cries
Sprang from sparrows finding scattered oats among interstices of the curb.
Ah-hah these metaphors—and Ah-hah these boys—among the police they were known
As the Dirty Dozen and their names took the front pages of newspapers
And two of them croaked on the same day at a “necktie party” … if we employ the metaphors of their lips.
Nowadays,  The Dirty Dozen  is best known as the title of a 1967 war film, about a fictional unit in WWII: 
 
 Sandburg is talking about a street gang, presumably fictional, although the famous Depression-era gangster John Dillinger was the  leader of a gang by that name  as a kid .
Alley Rats