Green’s Dictionary of Slang

shady adj.

1. stupid, strange.

[US]B.H. Hall College Words (rev. edn) 421: shady. [...] an epithet of depreciation, equivalent to Mild and Slow.
[US] ‘Sl. of Watts’ in Current Sl. III:2.
[US]Eble Campus Sl. Apr. 7: shady – odd, out of the ordinary: ‘Those kids who sit in front of the Post Office are shady.’.
[US]Prison Slang Mommyblogger 2 Mar. 🌐 Shady-ass motherfucker is obviously throwed off, nutted up, and off the chain. The man is plain shot-out.

2. obscene, smutty.

[US]Nat. Police Gaz. (NY) 18 Mar. 6/4: The manager [and] his friends laugh over a shady story they have Just finished listening to.
[UK]Mirror of Life 21 Apr. 14/4: [headline] Anything a Trifle ‘Shady’ is What the Public Cry For.

3. (US) surreptitious, secretive, self-contained.

[US]T. Wolff (con. mid-1950s) This Boy’s Life 127: Bobby had a very soft voice, and this made what he said seem confidential, even a little shady .
Urban Dict. 12 Apr. 🌐 A shady person is someone who does not keep it real; a person who hides out; a person who keeps things to themselves. Damn girl I ain't seen you in forever. You are shady as hell.
[US]N. Walker Cherry 26: ‘I was just thinking...that you’re shady’.

SE in slang uses

In compounds

shady language (n.)

(N.Z. prison) prison slang.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 163/1: shady language n. criminal jargon, prison argot.

In phrases

keep shady (v.)

(UK Und.) to act discreetly; to keep oneself (or someone else) hidden.

[US]Whip & Satirist of NY & Brooklyn (NY) 26 Nov. n.p.: Lydia, keep shady, Ike has no more $20 bills.
[US]J.M. Field Drama in Pokerville 81: I kep’ shady, Miss Fanny, bress de Lord, I did.
[US]N.Y. Eve. Express 19 Oct. 1/4–5: [Our reporter] found them [i.e. ‘the better sort of colored people’] disposed to ‘keep shady’ on the subject [i.e. John Brown’s insurrection], and it was hard to draw from them any opinion whatever on the matter at issue.
[US]N.E. Police Gaz. (Boston, MA) 18 Aug. 6/4: Keep shady [...] the eye of the avenger is upon you.
[UK]Hampshire Advertiser 27 Dec. 7/6: The great laconic philosopher, Shirke, says — ‘eep shady: if you see a sovereign on the ground, put your foot on it until no one is looking.
[UK]Leaves from Diary of Celebrated Burglar 102/1: The conductor [was] induced to keep ‘shady’ on the day when Jemmy was brought up on remand; consequently there being no one there to press the charge, Captain Grant was discharged.
[US]G.G. Hart E.C.B. Susan Jane 21: Arrah divil a word but what is true, / So let yez all keep shady.
[US]G. Davis Recoll. Sea-Wanderer 33: I had been specially cautioned to keep shady and [...] I remained close below.
F.R. Stockton Hundreth Man xii: I’ve promised to keep him shady [DA].
S.T. Clover Paul Travers’ Adventures 51: I guess I can fix you out if you hang around here, but keep shady [DA].
Leavenworth Post (KS) 21 Sept. 1/7: Mrs Adams’ husband, who keeps shady, [...] left town.