Pecker gag
From Wipipedia
A pecker gag is a kind of gag. It consists of a pecker (an object resembling a penis or dildo), large enough almost to fill the mouth, attached to a strap that buckles behind the head to hold it in place.
It is possible to have an inflatable pecker; in that case, a strap may not be needed, since it is difficult to eject an inflated gag even if it is not strapped in.
