Green’s Dictionary of Slang

1. anything insignificant.

1981		A. Dewlen Session 277: They haven’t done diddly-poo for me all year.
1989		H. Rawson Dict. of Invective (1991) 352: diddly shit. A measure of insignificance, applied to anything of little or no value; a.k.a. diddly poo, diddly squat, or just plain diddly.
2007		M. Burgess Bloodsong 192: The fact is they don’t know diddly-poo.

2. attrib. use of sense 1.

2004

B. Hale Murder My Tweet 60: Out you go — just like your no-good, diddly-poo, mockingbird friend!