Source : Webster's Revised Unabridged Dictionary (1913)

Pennyweight \Pen"ny*weight`\, n.
   A troy weight containing twenty-four grains, or the twentieth
   part of an ounce; as, a pennyweight of gold or of arsenic. It
   was anciently the weight of a silver penny, whence the name.

Source : WordNet®

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