Green’s Dictionary of Slang

1. the middle.

1969

S.T. Kendall Up the Frog.

2. an act of urination [= piddle n. (2)].

1959		D. Niland Big Smoke 138: What are ya doing there, ya old bastard, having a hi-diddle-diddle or burgling the joint?
2002		Pete’s Aussie Sl. Home Page 🌐 hi-diddle-diddle: a piddle.

3. a fiddle.

1969

S.T. Kendall Up the Frog.