When you no longer need to refer to someone by their last name.
Usally used when 1. two enemies (particuarly
Jocks) are trying to end a fight or if 2. an adult wants you to call them by their
first name.
Draco Malfoy: Hey Harry
Harry Potter: What so now we're on first name basis?
you: You have a beutiful house
Mrs. Smith
Mrs. Smith: Call me Wanda.
You:
Oookay that's
a little strange