Heavy metal Meaning, Definition & Usage

  1. noun a metal of relatively high density (specific gravity greater than about 5) or of high relative atomic weight (especially one that is poisonous like mercury or lead)
  2. noun loud and harsh sounding rock music with a strong beat; lyrics usually involve violent or fantastic imagery
    heavy metal music.

WordNet

Definitions
  1. (Mil.), large guns carrying balls of a large size; also, large balls for such guns. a type of rock music (1970's), with a hard beat, amplified electronically

Webster 1913