First class and Impugnable

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

Impugnable and first class are semantically related. In some cases you can use "Impugnable" instead an adjective phrase "First class".

Impugnable adjective - Subject to being discredited.

First class and impugnable are semantically related. Sometimes you can use "First class" instead an adjective "Impugnable".