Kettle is short for "Ma and Pa Kettle," i.e. a naive and uninformed casual traveler, as in, "I heard a Kettle with an economy ticket tell an FA, 'But I thought we could just sit anywhere. Aren't all the seats the same?'"

Related term: Gomer, short for "Gomer Pyle," meaning a louder and stupider Kettle, as in, "This Gomer boarded and said, 'Well, Golly, I wish I had a seat by the window because I like to keep it rolled down to get me some of that fresh air.'"