First class was a term used to describe a high tier of quality of one's travel or living arrangements.
In 2152, Trip Tucker apologized to Rellus Tagrim for having to settle him, Guri, and Renth in Enterprise NX-01's catwalk, saying they weren't first class accommodations, but Tagrim assured him they would do. (ENT: "The Catwalk")
When Harry Mudd was left behind by Captain James T. Kirk on Planet Mudd under the continued care of the Mudd androids in 2268, Kirk informed Mudd that "You'll stay here and provide a first class example to the androids of a Human failure. They'll learn by close observation how to avoid ones like you in the future." (TOS: "I, Mudd")