Source : Webster's Revised Unabridged Dictionary (1913)

Serbonian \Ser*bo"ni*an\, a.
   Relating to the lake of Serbonis in Egypt, which by reason of
   the sand blowing into it had a deceptive appearance of being
   solid land, but was a bog.

         A gulf profound as that Serbonian bog . . . Where
         armies whole have sunk.                  --Milton.