Green’s Dictionary of Slang

1. (Irish/UK juv.) a penis.

1981		(con. 1920s) P. Crosbie Your Dinner’s Poured Out! 219: As small as a mouse’s diddy.
1993		R. Doyle Paddy Clarke Ha Ha Ha 156: I held his nose and I pinched his diddy. [Ibid.] 168: Sinbad got a pulled diddy and a dead leg at the same time.

2. by ext. as term of abuse.

2009

T. Black Gutted 21: Had it loud enough to drown out the brewery diddies’ best work.