The Collaborative International Dictionary of English v.0.48:

Gruff \Gruff\ (gr[u^]f), a. [Compar. Gruffer (gr[u^]f"[~e]r);
   superl. Gruffest.] [D. grof; akin to G. grob, OHG. gerob,
   grob, Dan. grov, Sw. grof, perh. akin to AS. rc['o]fan to
   break, Z. reavc, rupture, g- standing for the AS. prefix ge-,
   Goth. ga-.]
   Of a rough or stern manner, voice, or countenance; sour;
   surly; severe; harsh. --Addison.
   [1913 Webster]

         Gruff, disagreeable, sarcastic remarks.  --Thackeray.
   -- Gruff"ly, adv. -- Gruff"ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gruffly
    adv 1: in a gruff manner; "`No,' he replied gruffly"