c47
In the film industry, it refers to a clothespin. Often used by the lighting department to attach gels to lights, keep cords out of the way, etc.
A: Hey new guy...hand me a c47.
B: *blank stare*
A: C47! *sigh*...a clothespin!
b: Oh.
B: *blank stare*
A: C47! *sigh*...a clothespin!
b: Oh.