Green’s Dictionary of Slang

peaky adj.

also peeky
[SE peak, to look sickly]

sickly-looking, by ext. weak, feeble.

implied in peakyish
[UK]‘Cuthbert Bede’ Little Mr. Bouncer 19: You pretend to be Æger, or sick and peaky, when you’re in robust health.
[UK]Sl. Dict.
[UK]Manchester Courier 25 June 5: A Texas Mother-In-Law [addressing a policeman] To think of this peaky, little, worthless, spindle-shanked, goggled-eyed whelp getting clear off.
[US]M.G. Hayden ‘Terms Of Disparagement’ in DN IV:iii 215: peaky, peeky, sickly. ‘She is looking peaky from a cold but otherwise well.’.
[UK]W. Pett Ridge Madame Prince 32: One of ’em looks a bit peaky.
[Aus]K.S. Prichard Haxby’s Circus 154: Mart decided Max was looking a bit peaky: the child really needed a change.
[NZ]D. Davin For the Rest of Our Lives 194: He remembered how when he was a child if a hen got sick – peaky his mother used to call it – she slunk aside from the rest.
[Aus]D. Stivens Scholarly Mouse and other Tales 46: Look a bit peaky, both of you. I’ll take you in hand. Long bouncy walks for both of you will be the cure.
[UK]H.E. Bates When the Green Woods Laugh (1985) 287: You look a bit peaky, Mr Jerebohm.
[UK]A. Ayckbourn Joking Apart II i: You’ve been very peaky lately. Can’t we have a proper holiday sometime?
[UK]T. Blacker Fixx 198: You’re looking a bit peaky these days.
[Scot]I. Welsh Trainspotting 247: You look a bit peaky. Probably just a touch of this little bug that’s doing the rounds.
[UK]T. Blacker Kill Your Darlings 230: You all right? [...] You look a bit peaky.

In derivatives