Green’s Dictionary of Slang

used to describe a police officer.

1905		F. Hutchison Philosophy of Johnny the Gent 79: ‘You know that big, flat-footed, rough-neck that used to drill a beat over by Clancy’s?’.
1925		D. Hammett ‘The Scorched Face’ Story Omnibus (1966) 89: He was shot by a bona fide, star-wearing, flat-footed officer of the peace.
1931		C.B. Yorke ‘Snowbound’ in Gangster Stories Oct. n.p.: ‘How’s the D.A. and all your flat-footed playmates?’.
1939		K. Tennant Foveaux 54: I’m goin’ to find that blasted flat-footed Hannam an’ polish his buttons wiv ’is teeth.
1947		G. Kersh Prelude to a Certain Midnight Bk I Ch. vi: A flat-footed old police constable who tramped a local beat.