When you go out and make friends but you were too wasted to get numbers or just too wasted and you'll never be friends in real life. "One night friends"
A polite way of saying "you're honestly terrifying and I'm worried a more direct rejection will cause you to break into my house and murder me in the night."
People on the receiving end of this phrase are expected to simply grow up and take the rejection like reasonable adults instead of whining like small children over the internet.
Person A: I like you.
Person B: I'm sorry, lets just be friends.
Person A: Oh ok. I can't say I'm not disappointed, but I understand that you are under no obligation to love me even if I was kind to you and we share a lot of the same interests, and I also understand that you are not obligated to give me a reason as to why you don't like me. I'll respect your decision because I actually have respect for you, and that respect is not and never was hingent upon whether or not we would have sex.
Leftover friends are friends of friends in school which are now your main friends due to the fact that your original 'main' friends have moved out of the area and left you with the....leftover friends. Can also apply to someone introduced at work by a friend which is now a better friend than the original.
Now that my friend Jill went off to college, I am stuck with my leftover friends she introduced me to.