The Collaborative International Dictionary of English v.0.48:

kopje \kop"je\, koppie \kop"pie\(k[o^]p"[i^]) n. [South African
   D., dim. of kop. See Kop.]
   A hillock; a small kop, especially a small hill rising up
   from the African veld. [South Africa]
   [Webster 1913 Suppl. + WordNet 1.5]

   Note: The colloqual Dutch pronunciation as here given is the
         usual one in South Africa.
         [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

kopje
    n 1: a small hill rising up from the African veld [syn: kopje,
         koppie]