Source : Webster's Revised Unabridged Dictionary (1913)

Stiffly \Stiff"ly\, adv.
   In a stiff manner.

Source : WordNet®

stiffly
     adv 1: in a stiff manner; "his hands lay stiffly" [syn: {stiff}]
     2: in a rigid manner; "the body was rigidly erect"; "ge sat
        bolt upright" [syn: {rigidly}, {bolt}]