The term simply means sandwich. Originating in the southeastern states of the U.S. the term is representative of a pervasively localized regional dialect. Other cross-over examples from this dialect include: fifty-leven, ho, and stank.
An exhausted southern mother to her son:
Boy, leave me alone and go make you a may-nayse (mayonnaise) sammich.
N.B. other sammich varieties include: pork & bean, jelly, pork chop, egg, butter (i.e. margarine), sugar, and collard green.