Weeaboo is a weird term, to be honest, and most people use it wrong. In fact, its whole usage is based on using is wrong. It's an insult, at best, in addition to being a term used to degrade one's standing within a community and make the speaker feel better about themselves. It's pretty much only used to bully others.

As for why some people use it, bullying is a reason, but it's likely for no other reason than that you like anime. You also mention that there's something between the two of you. The fact you need to mention this kinda tells me it's more on your end than his. He may like talking to you, but you're likely the one with more developed feeling for this other person.

I don't know what exists between the two of you, but I do know that you guys need to have a chat. If you don't like this word, let them know. If they stop, good. If they don't, consider finding a new friend who actually cares for you. You don't want to be around someone who can't respect your boundaries.

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u mean DIshonor before Death right ??? :P