The Collaborative International Dictionary of English v.0.48:

hep \hep\ (h[e^]p), n.
   See Hip, the fruit of the dog-rose.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

hep \hep\ (h[e^]p), a.
   Same as Hip, a., but older and now less frequently used.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

hep \hep\ (h[e^]p), interj.
   A call used by drill instructors to count cadence during
   marching; used identically to hut and hup.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

hip \hip\, a.
   1. Aware of the latest ideas, trends, fashions, and
      developments in popular music and entertainment culture;
      not square; -- same as hep.

   Syn: tuned in.
        [PJC]

   2. Aware of the latest fashions and behaving as expected
      socially, especially in clothing style and musical taste;
      exhibiting an air of casual sophistication; cool; with it;
      -- used mostly among young people in the teens to
      twenties.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Hip \Hip\, n. [OE. hepe, AS. he['o]pe; cf. OHG. hiufo a bramble
   bush.] (Bot.)
   The fruit of a rosebush, especially of the English dog-rose
   (Rosa canina); called also rose hip. [Written also hop,
   hep.]
   [1913 Webster]

   Hip tree (Bot.), the dog-rose.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hep
    adj 1: informed about the latest trends [syn: hep, hip, hip
           to(p)]

The Free On-line Dictionary of Computing (19 January 2023):

HEP

   High Energy (Particle) Physics.