The Collaborative International Dictionary of English v.0.48:

Casually \Cas"u*al*ly\, adv.
   Without design; accidentally; fortuitously; by chance;
   occasionally.
   [1913 Webster]

WordNet (r) 3.0 (2006):

casually
    adv 1: not methodically or according to plan; "he dealt with his
           course work casually"
    2: in an unconcerned manner; "glanced casually at the headlines"
       [syn: casually, nonchalantly]