Green’s Dictionary of Slang

1. the pit of the stomach.

1934		in Jevne & Purcell Joe Palooka [film script] I hit him right in the labonza! [HDAS].
1943		I. Wolfert Tucker’s People (1944) 97: ‘Quite a kick in the labonza,’ said Murray.
1955		Mad mag. Mar. 9: You’re going to get it in the labonza.
2000		P. Roth Human Stain 101: Coleman caught him in what Doc Chizner called ‘the labonz’.

2. the buttocks.

1943		cited in Wentworth & Flexner DAS (1975) 311/1: The day’s losses – $39,374! Quite a kick in the labonza.
1955		Mad mag. Feb. 7: Hoboy ist das kapitan gegungen zu getten it! ... Ja boy ... Right im der labonza!
1966		Trimble 5000 Adult Sex Words and Phrases.
1972		B. Rodgers Queens’ Vernacular.
1989–2003		R.O. Scott Gay Sl. Dict. 🌐.