Smotherbox (or smothering box) is a type of dungeon furniture similar to a queening stool. It may be used in face sitting, where the Dominant role sits upon the face the submissive, for the purpose of receiving oral sex from the sub.
A smotherbox has two openings in it. One is in a vertical side of the box for the neck of the person who has their head inside the smotherbox, the other is on top of the box to expose the face. The inside of a smotherbox is often padded to provide support for neck of the sub, and to prevent their head from moving.