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)]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "hep":
   alive to, appreciative of, apprised of, awake to, aware of,
   behind the curtain, behind the scenes, canny, cognizant of,
   conscious of, hep to, in the know, in the secret, informed of,
   knowing, let into, mindful of, nimble-witted, no stranger to,
   on to, privy to, quick, quick-witted, seized of, sensible of,
   sensible to, sharp, sharp-witted, slick, smart, streetwise,
   undeceived, wise to



The Free On-line Dictionary of Computing (30 December 2018):

HEP

   High Energy (Particle) Physics.