"made of lead," Old English leaden, from lead (n.1) + -en (2). The figurative sense of "heavy, oppressive, dull" is attested by 1570s. Related: Leadenly; leadenness.
Entries linking to leaden
lead n.1
heavy metal, Old English lead "lead, leaden vessel," from West Germanic *lauda- (source also of Old Frisian lad, Middle Dutch loot, Dutch lood "lead," German Lot "weight, plummet"), a word of uncertain origin. The name and the skill in using the metal seem to have been borrowed from the Celts (compare Old Irish luaide).
Figurative of heaviness at least since early 14c. American English slang lead balloon "dismal failure" attested by 1957, perhaps 1940s (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.
Meaning "graphite in a pencil" is from 1816 (see pencil (n.)). 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. White lead (1560s) was an old name for "tin."
As a name of a dull bluish-gray color, 1610s. From 1590s as figurative for "bullets." Lead oxide was much used in glazing, mirror-making, and pigments. In printing, "thin strip of type-metal (often lead but sometimes brass) used in composition to separate lines" from 1808, earlier space-line. Lead-poisoning is from 1848; earlier lead-distemper (1774).
-en 2
suffix added to nouns to produce adjectives meaning "made of, of the nature of" (such as golden, oaken, woolen), corresponding to Latin -anus, -inus, Greek -inos; from Proto-Germanic *-ina- (from PIE *-no-, adjectival suffix).
Common in Old, Middle, and early Modern English: e.g. fyren "on fire; made of fire," rosen "made or consisting of roses," hunden "of dogs, canine," beanen "of beans," baken "baked," breaden "of bread;" Wycliffe has reeden made of or consisting of reeds." The few surviving instances are largely discarded in everyday use, and the simple form of the noun doubles as adjective (gold ring, wool sweater). Some are used in special contexts (brazen, wooden).