A carpenter's square, drawn plainly: a crossbar top and bottom joined by a single upright — the exact tool a Bronze Age craftsman used to true a beam or square a wall. From that one tool grew the word for the labor itself: to work, a craft, a trade.
A piece of culture
Scholars, farmers, workers, merchants Imperial China ranked its people in four classes, 士农工商: scholars (士) at the top for guarding knowledge, then farmers (农) who fed the empire, then craftsmen (工) who built and made things, and merchants (商) last of all — distrusted for profiting from others' labor rather than producing anything themselves. The ranking shaped careers for two thousand years: the civil service exams could lift a farmer's son to the top, while even a rich merchant's son ranked below him on paper. Today 工人 (gōngrén), "worker", and 工厂 (gōngchǎng), "factory", put the same ancient tool to work in a very different China.
Use it in a real word
手工
shǒugōng · sau2 gung1
handmade; handicraft — literally "hand-work"; you already know 手