Mid 19th century; earliest use found in The Morning Star. Probably from either muff or muff (although the verb is first attested slightly later) + glass.
Definition of muff glass in US English:
muff glass
noun
historical
Glass which has been muffed or muffled.
Origin
Mid 19th century; earliest use found in The Morning Star. Probably from either muff or muff (although the verb is first attested slightly later) + glass.