How old is the phrase “Good Shabbos”? I don’t know but as early as 1426, we know that Maharil said it on a regular Shabbos. When his wife died in that year (a year before his passing), on that Shabbos he did not bless the children in his community and did not — as he normally did — wish people “שבת טוב,” which in this Hebrew work is the equivalent of the Yiddish, “Gut Shabbos.”
