Genitive singular -- evidently extracted from somewhere -- of a feminine noun which has two related senses: a condition of decay/putrefaction/rot (cf. under epsilon 3711) and a snake whose bite causes the condition; see generally LSJ s.v. In the latter sense there is a instance of the genitive in Nicander, Theriaca 320, which prompted scholiastic comment and probably lies behind the present entry also.
For the related verb sh/pw, see sigma 350.