The Collaborative International Dictionary of English v.0.48:

Supercilious \Su`per*cil"i*ous\, a. [L. superciliosus, fr.
   supercilium an eyebrow, pride; super over, + cilium an
   eyelid; probably akin to celare to conceal. Cf. Conceal.]
   Lofty with pride; haughty; dictatorial; overbearing;
   arrogant; as, a supercilious officer; asupercilious air;
   supercilious behavior. -- Su`per*cil"i*ous*ly, adv. --
   Su`per*cil"i*ous*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

superciliously
    adv 1: with a sneer; in an uncomplimentary sneering manner; "`I
           don't believe in these customs,' he said sneeringly"
           [syn: sneeringly, superciliously, snidely]