The Collaborative International Dictionary of English v.0.48:

Siss \Siss\, v. i. [Of imitative origin; cf. D. sissen, G.
   zischen.]
   To make a hissing sound; as, a flatiron hot enough to siss
   when touched with a wet finger. [Colloq. U. S.; Local, Eng.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Siss \Siss\, n.
   A hissing noise. [Colloq. U. S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

siss
    v 1: express or utter with a hiss [syn: hiss, sizz, siss,
         sibilate]
    2: make a sharp hissing sound, as if to show disapproval [syn:
       hiss, siss, sizz, sibilate]