What is "exception that proves the rule"

n. 1 A form of argument in which the existence of a counterexample to a rule is used to demonstrate the fact that a rule exists. 2 (context idiomatic English) The rare occurrence of a counterexample to a rule, used to underscore that the rule exists

“The exception [that] proves the rule” is a saying whose meaning has been interpreted or misinterpreted in various ways. Its true, or at least original, meaning is that the presence of an exception applying to a specific case establishes (“proves”) that a general rule exists. For example, a sign that says “parking prohibited on Sundays” (the exception) “proves” that parking is allowed on the other six days of the week (the rule). A more explicit phrasing might be “the exception that proves the existence of the rule.”

An alternative explanation often encountered is that the word "prove" is used in the archaic sense of "test". Thus, the saying does not mean that an exception demonstrates a rule to be true or to exist, but that it tests the rule. In this sense it is usually used when an exception to a rule has been identified: for example, Mutillidae are wasps without wings, and therefore are an exception that proves (tests) the rule that wasps fly. The explanation that "proves" really means "tests" is, however, considered false by some sources.