serious (adj.)

Old English hefig "heavy, having much weight; important, grave; oppressive; slow, dull," from Proto-Germanic *hafiga "containing something; having weight" (source also of Old Saxon, Old High German hebig, Old Norse hofugr, Middle Dutch hevich, Dutch hevig), from PIE root *kap- "to grasp." Jazz slang sense of "profound, serious" is from 1937 but would have been comprehensible to an Anglo-Saxon. Heavy industry recorded from 1932. Heavy metal attested by 1839 in chemistry; in nautical jargon from at least 1744 in sense "large-caliber guns on a ship."

While we undervalue the nicely-balanced weight of broadsides which have lately been brought forward with all the grave precision of Cocker, we are well aware of the decided advantages of heavy metal. [United Services Journal, London, 1830]

As a type of rock music, from 1972. Most other Germanic languages use as their primary word for this their equivalent of Middle English swere, Old English swær "heavy, sad; oppressive, grievous; sluggish, inactive, weak" (but never in a physical sense; see serious); for example, Dutch zwaar, Old High German suari, German schwer. The English word died out in the Middle Ages.