Green’s Dictionary of Slang

1. walking carelessly.

1981		M. Baker Nam (1982) 161: I went diddy-bopping out there with them.
1989		(con. 1968) W.E. Merritt Where the Rivers Ran Backward 8: They accused me of diddly bopping. [Ibid.] 9: Sergeants treat diddly bopping as a moral flaw.
1990		G.R. Clark Words of the Vietnam War 143/2: Diddy-bopping in Vietnam was used to describe anyone walking in a careless, unattentive, indifferent, reckless or unconcerned manner. Diddy-bopping in the field could prove most hazardous.

2. living as a teen hoodlum; also attrib.

1962		(con. 1953–7) L. Yablonsky Violent Gang (1967) 81: All this diddley-boppin’ bullshit is just a waste, man.
1972		E. Grogan Ringolevio 446: Gangs of young, diddy-boppin’, black bloods.