A sort of picture-frame. The middle is cut out to the size of the picture, and the border or edge is embossed, so as to present a raised margin. The passe-partout and picture, being backed and faced with a glass, are held together by an edging of paper which shows on the glass face. The word means something to “pass over all.”
A master-key is also called a passe-partout (a pass through all the rooms).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894