Green’s Dictionary of Slang

gearbox n.

1. the buttocks, the posterior.

[UK]H. Daley This Small Cloud (1986) 195: [W]hen I came to present the brushes, he as good as told me to stuff them up my gearbox [Simes:DLSS].
[Can]J. Herfbert Fortune & Men’s Eyes 47: Queenie: It’s Catso-Ratso, your old gearbox buddy who’s got the greenies [Simes:DLSS].
[US]Leyland & Juddell (eds) Hot Acts 212: When I pressed my cock against his body, he began to say personal things about how he felt, how gearbox wanted love juice [Simes:DLSS].

2. the female genitals.

[UK]Partridge DSUE (8 edn).

3. see gear box under gear n.