Simp

Basically someone who really really really likes you for no apparent reason whatsoever unless they tell you that reason. Or someone who is always flirting with you, hangs out with you often, or always is around you all the time. Especially if they are dating or crushing on you if they've known for you a while.

A: Bro, there's this girl I like. Like...a lot and I don't even know why. I always like hanging out with her, and she flirts with me a lot, man.

B: Bro, you both are such a simp for each other.

A: Hey, can I have your number? Your super cool and amazing, I'd really like to hang out sometime.

B: Are you a simp or something? Get to know me first at least!