Broken in and First class

Broken in adjective - Tamed or trained to obey.

First class and broken in are semantically related. In some cases you can use "First class" instead an adjective phrase "Broken in".

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

Broken in and first class are semantically related. Sometimes you can use "Broken in" instead an adjective phrase "First class".