Pack Heat Meaning

(idiomatic) To carry one or more handguns on one's person, especially in a concealed manner.

Example: 1995, David Van Biema, "Peekaboo: The New Detector," Time, 27 Mar.:
  If the current trend toward relaxation of concealed-weapons laws continues, soon the most important gun restrictions may involve destinations: the states will probably allow various venues—public gathering places, offices, stores—to ban firearms on the premises. The only problem will be how to know who is packing heat.