shady (adj.)

Middle English schade, Kentish ssed, from late Old English scead "partial darkness; shelter, protection," also partly from sceadu "shade, shadow, darkness; shady place, arbor, protection from glare or heat," both from Proto-Germanic *skadwaz (source also of Old Saxon skado, Middle Dutch scade, Dutch schaduw, Old High German scato, German Schatten, Gothic skadus), from PIE *skot-wo-, from root *skoto- "dark, shade." 

shade, shadow, nn. It seems that the difference in form is fairly to be called an accidental one, the first representing the nominative & the second the oblique cases of the same word. The meanings are as closely parallel or intertwined as might be expected from this original identity, the wonder being that, with a differentiation so vague, each form should have maintained its existence by the side of the other. [Fowler]

Figurative use in reference to comparative obscurity is from 1640s. Meaning "a ghost" is from 1610s; dramatic (or mock-dramatic) expression "shades of _____" to invoke or acknowledge a memory is from 1818, from the "ghost" sense. Meaning "lamp cover" is from 1780. Sense of "window blind" first recorded 1845. Meaning "cover to protect the eyes" is from 1801. Meaning "grade of color" first recorded 1680s; that of "degree or gradiation of darkness in a color" is from 1680s (compare nuance, from French nue "cloud"). Meaning "small amount or degree" is from 1782.

Proto-Indo-European root meaning "dark, shade." 

It forms all or part of: nightshade; scotoma; shade; shadow; shady.

It is the hypothetical source of/evidence for its existence is provided by: Greek skotos "darkness, gloom;" Albanian kot "darkness;" Old Irish scath, Old Welsh scod, Breton squeut "darkness," Gaelic sgath "shade, shadow, shelter;" Old English scead "partial darkness," sceadu "shade, shadow, darkness," Dutch schaduw, German Schatten, Gothic skadus "shadow."