The magical door to another dimension where everyone looks like you when you look at them and copy's you.
The sad part is us mere mortals cannot break the barrier that is the other dimension so we are forced to look and copy our poor doppelgangers.
An archive site or web site which keeps a copy of some or all files at another site so as to make them more quickly available to local users and to reduce the load on the source site.
A nice way to comeback to a roast if you can't defend yourself or just want to roast back. When you say mirror, it's like they are looking in the mirror when they say it, so it goes back to them.
A mirror is someone who really doesnt have a personality of their own so they find someone to copy and mimic them. This can range from clothing to speech and even hobbies. You might think that imitation is the sincerest form of flattery but in this case it isnt
"Dude why is that guy wearing MY t shirt how did he even get it?"
"I dont know man he just a mirror."