Green’s Dictionary of Slang

(US campus) risky, dangerous.

1975		G. Underwood ‘Razorback Sl.’ in AS L:1/2 66: sketch adj Dangerous, risky.
2004		Eble Campus Sl. Apr. 6: sketchy/sketch – out of the ordinary, unfamiliar, perhaps dangerous: That side of town is sketch. I’m not driving through there.
2004		J.A. Juarez Brotherhood of Corruption 2: [T]he sketchy parts of Chicago where drugs, gangs, violence, poverty, and cheap rents flourish.
2012		M. Lacher On the Bro’d 268: Dudes [...] with shady vans [...] looking for some sketch-ass business.
2014		B. Kroeber ‘Be My Alibi’ in ThugLit Sept. [ebook] Our soon-to-be arrangement is at least a teensy bit sketch, right?