The Collaborative International Dictionary of English v.0.48:

Incognito \In*cog"ni*to\, n.; pl. Incognitos. [See
   Incognito, a.]
   [1913 Webster]
   1. One unknown or in disguise, or under an assumed character
      or name.
      [1913 Webster]

   2. The assumption of disguise or of a feigned character; the
      state of being in disguise or not recognized.
      [1913 Webster]

            His incognito was endangered.         --Sir W.
                                                  Scott.
      [1913 Webster]