(1)A person or situation that seems a little off, could possibly lead to trouble. (2) Just being said to piss someone off by calling them a sketch and having everyone else do the same until they flip the fuck out.
(1) Whoa dude this deal is kinda sketch maan.
(2) Kevin: Hey Chris why you being sketch?
Charles: Ya you fucking sketch
Tim, Lance, Eric, and Rick: SKETCH!!!
Chris: FUCK YOU GUYS IM NOT SKETCH YOUR SKETCH!!!
Kevin: (Whispers) ya ok sketch
A person who is dumb, not cool, fat, gay, bi sexual, sketch asexual, Adam Lambert, Clay Aiken, Knickelback, Thomas Swartz, Every Member of Hinder and Buckcherry, Ben Rothlesberger, Etc.. Etc..
If you're in traffic yell out your window "Move it HEADFUCK!!!" with a slightly German accent people will most likely move or try to fight you. Or you could be just walking down the road and scream "HEADFUCK HEADFUCK HEADFUCK" and people will believe you have tourettes.