The Collaborative International Dictionary of English v.0.48:

Missis \Mis"sis\, n.
   A mistress; a wife; -- so used by the illiterate. --G. Eliot.
   [1913 Webster]

WordNet (r) 3.0 (2006):

missis
    n 1: informal term of address for someone's wife [syn: missus,
         missis]