A Jancy is a "female best friend" of your current boyfriend. They will spend all of their time with him and will ultimately steal your boyfriend and possibly threaten your feminity as well.
A Jancy will either act super nice and normal around you or the exact opposite.
They are commonly found in rebound relationships. Though, your boyfriend may leave you for legit reasons the Jancy will always be your boyfriend's safety net which builds on her attraction factor over yours.
Kate: Damn girl! Your boyfriend moved on fast! Who's the new girl now?!
Diana: Ugh! Who cares what her name is! Jake wouldn't shut up about her when we were dating! Seriously, she's a total Jancy
Tim: Hey! You wanna chill this weekend?
Jake: Nah, I'm chilling with my Jancy
. We're going to the movies at 2!
Tim: In the morning? That's late. Watch out, bro. I don't know if your girl will like that.
Jake: Oh, she doesn't know.