My first thought was that the term could’ve predated Hawaii becoming a state, which only happened in 1959. But that seems to be false; the term only started get

My first thought was that the term could’ve predated Hawaii becoming a state, which only happened in 1959. But that seems to be false; the term only started getting popular around 1967: https://books.google.com/ngrams/graph?content=Lower+48&year_start=1800&year_end=2019&corpus=26&smoothing=3&direct_url=t1%3B%2CLower%2048%3B%2Cc0 https://books.google.com/ngrams/graph?content=Lower+48&year_...