Wiktionary
n. (context often in the plural English) Any of a group of friends who are often in each other’s company, and appear to be inseparable
Usage examples of "fast friend".
Mother did not say much, but I am sure that at heart she is very grateful, for the earl himself came down to the Rectory and spoke warmly about us, and said that he should always be our fast friend, because we had given his cousin some help when he was roughly pressed by the Spaniards.
Much more probably, this peace-loving warlord of Cousin Arthur's would make di Bolgia a fast friend, join the two condottas into one, and then I'd have a real threat, a mobile threat, ready to leap out of Munster at my very throat at any time.
My hostile great-aunt was now my closest relation, but the other priestesses were kind to me, and among the maidens, Aelia was my fast friend.
He was a swaggering bantam rooster of a youth, too young and too hot-blooded for Ned's taste, though a fast friend of Catelyn's brother, Edmure Tully.