First class and Vet

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

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

Vet noun - A person with long experience in a specified area.

First class and vet are semantically related. Sometimes you can use "First class" instead an adjective "Vet".