First class and High hat

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

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

High hat noun - A man's hat with a tall crown; usually covered with silk or with beaver fur.

First class and high hat are semantically related. Sometimes you can use "First class" instead an adjective phrase "High hat".