The Collaborative International Dictionary of English v.0.48:

Fog \Fog\ (f[o^]g), n. [Cf. Scot. fog, fouge, moss, foggage rank
   grass, LL. fogagium, W. ffwg dry grass.] (Agric.)
   (a) A second growth of grass; aftergrass.
   (b) Dead or decaying grass remaining on land through the
       winter; -- called also foggage. [Prov.Eng.]
       --Halliwell.

   Note: Sometimes called, in New England, old tore. In
         Scotland, fog is a general name for moss.
         [1913 Webster]