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
adj. Descriptive of an uphill gardener who's so far in the closet he's found Narnia, taken out a mortgage there and spends his days fraternising with the local populace.
"I don't care if he was married to the bird off Dawson's Creek, that Tom Cruise is so far in the closet he's on first name terms with Mr Tumnus".