Green’s Dictionary of Slang

lunchbox n.

1. (US black) the stomach.

[US]C. Major Juba to Jive.

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

[US]M. Leyner Et Tu, Babe (1993) 16: ‘Look at the lunch-pack on this guy,’ she said, appreciating my scrotum.
[UK]Guardian Editor 5: If I had a lunchbox of that diminutive size, I would not personally be flashing it on TV.
Twitter 16 Mar. 🌐 [Simon] Heffer quotes in a footnote a 2012 @telegraphobits piece about David Metcalf who was excused wearing shorts at Eton ‘owing to a larger-than-life lunch box’.

3. the female genitals.

[US]‘Randy Everhard’ Tattoo of a Naked Lady 45: She crossed her powerful legs over me, crushing me deeper into her lunchbox.

4. see lunch bag under lunch n.

In compounds

lunchbox lancer (n.)

a male homosexual.

[UK]Roger’s Profanisaurus in Viz 87 Dec. n.p.: lunchbox lancer n. archaic Medieval term for a coco shunter (qv). First used by King Arthur to describe the French army. He wrote: ‘Fear not the French, for their knights are without heart, and their King without wisdom But keep thine backs as to the wall, for amongst their number thou shalt find more than a few lunchbox lancers, I can tell you.’.

In phrases

open one’s lunch(box) (v.)

to break wind; thus question who opened their lunch(box)?

[Aus]B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 23: Cripes, what a whiff! Who opened their lunch? [...] Like who farted?
[NZ]McGill Dict. of Kiwi Sl. 70/2: lunch box bum, in phr. open your lunch box, to fart, aka drop your lunch box.
[UK]Roger’s Profanisaurus in Viz 87 Dec. n.p.: open your lunchbox euph. To float an air biscuit (qv) resembling yesterday’s egg sandwiches.
OnLine Dict. of Playground Sl. 🌐 lunch-box (open the ... ) v. fart.
[Ire]P. Howard Curious Incident of the Dog in the Nightdress 5: Oisinn [...] sniffs the air and goes, ‘Who opened their lunch?’.
your lunchbox is open

your fly is open.

[US]D. Lebofsky Lex. of Phila. Metropolitan Area n.p.: Close the stable or the horse’ll get out, who died—your flag is at half-mast, your lunch box is open, flies cause disease—keep yours closed, what do birds do?—your fly is open.