1805, "having the form or character of a sketch," from sketch (n.) + -y (1). Colloquial sense of "unsubstantial, imperfect, flimsy" is from 1878, perhaps via the notion of "unfinished." Related: Sketchily; sketchiness.
A street derelict who may be a villain: Perp: also a mutt, a mope, a skid, or a skell
[1980s+ New York City police; ultimately fr Dutch schelm, ''scoundrel, villain,'' found in English as skelum by 1611]