firebox

fire +‎ box. In the sense of a redheaded woman, from box (“vagina”).

firebox (plural fireboxes)

1. (rail transport) The chamber of a steam engine, or a steam locomotive, in which the fuel is burned.
   • 1951 April, Stirling Everard, “A Matter of Pedigree”, in Railway Magazine, number 600, page 273:

     The firebox married to Britannia's boiler is not, however, in the Doncaster tradition, notwithstanding that it is comparable in dimensions to that of the "V2." […] Britannia's firebox would appear to have derived from those of the Bulleid Pacifics, which it closely resembles.

2. The part of a fireplace where the fuel is burned.
3. (vulgar) A redheaded woman or her red pubic hair.

Diagram of a steam locomotive. Firebox is #1.

• red cunt hair