Acid test

The metaphorical or idiomatic use of this expression dates from the mid-19th century in the form of something or other 'undergoing or passing the acid test,' which means that it has undergone or passed a rigorous, crucial test, hence proving its worth, excellence, workability or performance. In short, it has passed muster. Its origin, however, is a complex one. It derives from chemistry, and the assaying or 'touching' of gold and other precious metals. The acid test followed on from the discovery that aqua fortis or nitric acid, as it became known, dissolves most metals, but not gold. Therefore, gold passed 'the acid test.' Thereafter, passing the acid test became the metaphor and idiom it is today, as described above. See also touchstone and litmus test.