Green’s Dictionary of Slang

1. (also dum-dumb, dumb-dumb) a fool, an idiot, a general term of abuse; also as adj.

1937		J. Steinbeck Of Mice and Men 86: A nigger an’ a dum-dum and a lousy ol’ sheep.
1949		Monteleone Criminal Sl. (rev. edn) 76: dum-dumb A shiftless no account drunkard.
1967		L. Bruce Essential Lenny Bruce 58: Get back to the blackboard, dum-dum. [Ibid.] 206: He’s yelling like a real dum-dum.
1972		M. Rodgers Freaky Friday 103: Listen, that’s enough of your dumb-dumb jokes.
1974		‘P.B. Yuill’ Hazell Plays Solomon (1976) 7: What was she doing married to this dum-dum?
1978		T. O’Brien Going After Cacciato (1980) 15: Dumb-dumb. Right, sir? Dumb as a dink.
1986		P. Slabolepszy ‘Boo to the Moon’ in Mooi Street (1994) 132: Bunch a’ bladdy dum-dums.
1989		H. Rawson Dict. of Invective (1991) 130: Dumb makes an excellent prefix or intensifier. For example: dumb-ass, dumbbell, dumb blonde, dumb bunny, dumb cluck, dumb Dora, dumb-dumb.
2001		(con. 1975–6) E. Little Steel Toes 65: Me and dumdum here will be right behind ya.

2. the penis.

1970

E. Tidyman Shaft 33: The little cocksucker would probably whip his dum-dum right out of the socket.