Other forms: sketchiest; sketchier
Something sketchy is incomplete: it includes the major points but lacks detail. If a political candidate avoids going into detail about her political views, you might say her platform is sketchy.
A sketch is a quick drawing that doesn't have a lot of detail. That definition should help you remember that sketchy things are incomplete. A sketchy speech from the President will outline some important ideas but be short on specifics. A sketchy plan for the weekend would be "Let's go out to eat." That's sketchy because it has the general idea but no specifics, such as the time or place.