|a rigorous and conclusive test to establish worth or value: the play passed the critic's acid test|
|[C19: from the testing of gold with nitric acid]|
|a scrap or morsel of food left at a meal.|
|a stew of meat, vegetables, potatoes, etc.|
A decisive trial to determine worth or quality, as in Exposure to brilliant sunlight is the acid test for showing this fabric won't fade. Alluding to a 19th-century chemical test for distinguishing gold from other metals, this term was used figuratively by the early 1900s.