The meaning "disreputable" (1862) might be from or reinforced by the earlier university-slang sense of "of questionable merit, unreliable" (1848, perhaps on the notion of "such as cannot bear the light"). Related: Shadily; shadiness. Old English had sceadlic "shady" ('shadely'); the Elizabethans also had shadeful. Colloquial on the shady side of "older than" (a specified age) is by 1808.
updated on July 22, 2022