Big-time and First class

Big-time adverb - To a large extent or degree.

First class and big-time are semantically related In some cases you can replace phrase "First class" with "Big-time", this phrases are similar.

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

Big-time and first class are semantically related in eminent topic. Sometimes you can use "Big-time" instead an adjective phrase "First class".