heavy metal, Old English lead , from West Germanic *loudhom (cf. Old Frisian lad , Middle Dutch loot , Dutch lood "lead," German Lot "weight, plummet"). The name and the skill in using the metal seem to have been borrowed from the Celts (cf. Old Irish luaide ), probably from PIE root *plou(d)- "to flow."
Figurative of heaviness since at least early 14c. Black lead was an old name for "graphite," hence lead pencil (1680s) and the colloquial figurative phrase to have lead in one's pencil "be possessed of (especially male sexual) vigor," attested by 1902. Lead balloon "a failure," American English slang, attested by 1957 (as a type of something heavy that can be kept up only with effort, from 1904). Lead-footed "slow" is from 1896; opposite sense of "fast" emerged 1940s in trucker's jargon, from notion of a foot heavy on the gas pedal.