The Collaborative International Dictionary of English v.0.48:

Pennyweight \Pen"ny*weight`\, n.
   A troy weight containing twenty-four grains, or the twentieth
   part of a troy ounce; 1.555 grams; as, a pennyweight of gold
   or of arsenic. It is abbreviated dwt or pwt. It was anciently
   the weight of a silver penny, whence the name.
   [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

pennyweight
    n 1: a unit of apothecary weight equal to 24 grains

Bouvier's Law Dictionary, Revised 6th Ed (1856):

PENNYWEIGHT. A troy weight which weighs twenty-four grains, or one-twentieth
part of an ounce. Vide Weights.