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.
[1990s+] a male homosexual.
[1980s+] to break wind; thus question who opened their lunch(box)?
[1970s] your fly is open.