The Collaborative International Dictionary of English v.0.48:

Rackarock \Rack"a*rock`\ (r[a^]k"[.a]*r[o^]k`), n. [Rack to
   stretch, strain + a + rock.]
   A Sprengel explosive consisting of potassium chlorate and
   mono-nitrobenzene.
   [Webster 1913 Suppl.]