Bone-box

Bone-box is Cant for one's mouth.

Example: "Shut your bleedin' bone-box, Berk!"

Source: Planewalkers Handbook

"Bone-box" is a cant term for mouth usually used by residents of Sigil.
For example "Quit rattlin' your bone-box." is cant for "Shut up" and
likewise "He's always rattling his bone-box" can be used to describe
someone who talks too much for his own good.

Sources:
Planescape Campaign Setting