Kirst is an expression used to describe someone who pretends they have a pet cat to be cool and fit in with society.
A 'kirst' can usually be detected because they unneccarily bring up their make believe cat into a conversation. This is all part of their attempt to make their elaborate lie believable.
Girl: Wow that rainbow looks so lovely
Kirst: yes me and my cat looked at YouTube videos of rainbows last night Girl: stop being such a kirst. I accept you without a cat!
Kirst: I do have a cat called Harvey!
Kirito: *is right* oh shit I completely forgot
Asuna: What is it, did we miss something Kirito: The Kirito is always right foundation, grand reopening
Asuna: *Stabs Kirito in eye*