Definition: The terms fast friends refers to two people who become close pals as soon as they meet. Usually friendship takes time to develop, but with fast friends it is as if the people involved have known each other their entire lives. They have many things in common, similar personalities, and get along very well from the start. Fast friends usually remain close for a long time.
The term applies to how quickly the friendship developed rather than whether or not the two people are best friends (or BFFs).
Examples:
"I met Sally and within minutes we were fast friends."
"John and I couldn't have more in common. We've been fast friends from the start."
