Origins
A sketch was, in its earliest Italian sense, a splash. The word comes from schizzo, which meant both 'a splash of liquid' and 'a quick drawing' — the two ideas merging in the image of ink thrown spontaneously onto paper. Behind the Italian lies Greek σχέδιος (schédios), meaning 'done offhand' or 'improvised'.
The Greek root reveals the sketch's philosophical position. Something schédios was done approximately — close to the target but not quite there. A sketch is not a failure; it is a deliberate near-miss, an exploration that stops before precision arrives.
Development
The word reached English in the 1660s via Dutch schets, carried by the vibrant artistic trade between the Netherlands and England. Dutch Golden Age painters valued the sketch as a working tool — a way to capture light, composition, and movement before committing to oil on canvas.
Modern English has stretched the word well beyond art. A comedy sketch is a quick, improvised scene. A biographical sketch is a brief outline of a life. And sketchy — meaning 'unreliable' or 'suspicious' — preserves the original sense perfectly: something sketchy is rough, unfinished, not to be trusted. The splash of ink has become a measure of credibility.