Green’s Dictionary of Slang

1. the King George V Silver medal Jubilee and the Edward VIII Coronation medals or ribbons or the 1918 Victory and Overseas medals or ribbons, which are invariably worn together.

1937–84

Partridge DSUE (1984) 770: 1918.

2. (orig. US) a pair of stupid, bungling men; also as adj.

1917		E.E. Cummings in Dupee & Stade Sel. Letters (1972) 4 June 26: By failing to get up at the hour of 5.45 AM I escaped departing with the bums mutts and jeffs (not to say ginks, slobs, and punks) who came over with us.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 212: The two of them looking like Mutt and Jeff.
1955		B. Schulberg On the Waterfront (1964) 42: Runty Nolan and big, bull-voiced Moose McGonigle, a Mutt-and-Jeff combo who did a lot of drinking and clowning together.
1979		‘Iceberg Slim’ Airtight Willie and Me 58: A pair of sharply dressed Mutt and Jeff Italian dudes.
1998		(con. 1986) G. Pelecanos Sweet Forever 51: Mutt and jeff were back on the couch.

3. (US) foolish conversation.

1937

M. Huxley in A. Huxley Letters (1969) 426: A sort of Mutt and Jeff on war and peace and religion and so on .

4. (US Und.) a pair of (usu. police) interrogators who work as ‘nice’ and ‘nasty’, in order to elicit information; also as adj.

1964		Wash. Post 30 Aug. A13/1: One of the FBI interrogation techniques is the old ‘Mutt and Jeff’ routine taught by Army Intelligence. It operates this way: One agent abuses his subject, firing tough questions and snide accusations. Then the other agent, who acts as the higher-ranking of the two, reprimands his partner and treats his subject with kid gloves.
1975		E. Torres Carlito’s Way 15: Big Jeff from the ‘Mutt and jeff’ detective team from the Twenty-Third.
1988		D. Woodrell Muscle for the Wing 140: You changed parts [...] First he was jeff and you was Mutt. Now he’s comin’ on Mutt and you get to be Jeff.
1991		O.D. Brooks Legs 34: The typist looked up, nodded at Mutt and Jeff and went back to poking the keys with index fingers.