The Collaborative International Dictionary of English v.0.48:

Facetious \Fa*ce"tious\, a. [Cf. F. fac['e]tieux. See
   Faceti[ae].]
   1. Given to wit and good humor; merry; sportive; jocular; as,
      a facetious companion.
      [1913 Webster]

   2. Characterized by wit and pleasantry; exciting laughter;
      as, a facetious story or reply. -- Fa*ce"tious*ly, adv.
      -- Fa*ce"tious*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

facetiously
    adv 1: not seriously; "I meant it facetiously" [syn:
           facetiously, jokingly, tongue-in-cheek]