Definition of "leaden"

Where but to think is to be full of sorrow / And leaden-eyed despairs, / Where Beauty cannot keep her lustrous eyes, / Or new Love pine at them beyond to-morrow.

1819, John Keats, “Ode to a Nightingale”, in Lamia, Isabella, the Eve of St. Agnes, and Other Poems, London: […] [Thomas Davison] for Taylor and Hessey, […], published 1820, stanza 3, page 109

[...] if man beThe passive thing you say, I should not seeMuch harm in the religions and old saws(Tho' I may never own such leaden laws)Which break a teachless nature to the yoke.

1818-1819, Percy Bysshe Shelley, Julian and Maddalo

"It was at the end of February..., when the world was cold..., when icy rains fell from the leaden skies in continual drizzling showers."

1999: Stardust, Neil Gaiman, page 31 (2001 Perennial paperback edition)