The Collaborative International Dictionary of English v.0.48:

Plong'ee \Plon`g['e]e"\, n. [F. See Plunge.] (Mil.)
   A slope or sloping toward the front; as, the plong['e]e of a
   parapet; the plong['e]e of a shell in its course. [Sometimes
   written plonge.]
   [1913 Webster]