The Collaborative International Dictionary of English v.0.48:

Apollyon \A*pol"ly*on\, n. [Gr. ? destroying, fr. ?, ?, to
   destroy utterly; ? from, entirely + ? to destroy.]
   The Destroyer; -- a name used --(Rev. ix. 11) for the angel
   of the bottomless pit, answering to the Hebrew Abaddon.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Apollyon
   destroyer, the name given to the king of the hosts represented
   by the locusts (Rev. 9:11). It is the Greek translation of the
   Hebrew Abaddon (q.v.).


Hitchcock's Bible Names Dictionary (late 1800's):

Apollyon, a destroyer