Green’s Dictionary of Slang

lunchbox n.

1. [1970s] (US black) the stomach.

2. [1980s+] (also lunchpack) the male genitals, esp. when large and prominent beneath tight shorts or trousers.

3. [2000s] the female genitals.

4. see lunch bag under lunch n.

In compounds

lunchbox lancer (n.)

[1990s+] a male homosexual.

In phrases

open one’s lunch(box) (v.)

[1980s+] to break wind; thus question who opened their lunch(box)?

your lunchbox is open

[1970s] your fly is open.