means A rough, quickly made drawing or outline that captures the basic idea of something without the finished detail.

from From the Dutch 'schets,' which came from the Italian 'schizzo,' meaning a rough draft or splash — itself traced back to the Latin 'schedium,' an extemporaneous or impromptu thing, related to the Greek 'skhedios,' meaning 'done offhand' or 'on the spot.' So a sketch has always meant something dashed off in the moment — the picture before the polish. The slang sense of 'sketchy,' meaning dodgy or unreliable, is a much later cousin, born from the idea of something only half drawn-in and therefore not to be fully trusted.