The Collaborative International Dictionary of English v.0.48:

Hypocoristic \Hyp`o*co*ris"tic\, a. [Gr. ?; ? under + ? to
   caress.]
   Endearing; diminutive; as, the hypocoristic form of a name.
   [1913 Webster]

         The hypocoristic or pet form of William. --Dr. Murray.
   [1913 Webster]