A Japanese honorific. It means "upperclassman," "senior," or the like. It has been appropriated for use in America and the West to mean "obsession" by weaboos. They use this word to attract their Tumblr/DeviantArt/etc. crushes or art seniors to give them art tips or recognition when perhaps they didn't deserve it.
Girl 1: *thinking* I really like this person. But they won't talk to me.
Girl 1: *typing* ArkhamGeek1 is my senpai! I couldn't do art like I do without them!
Girl 1: *thinking* With that on my blog page, they'll see how much I like them.
Girl 2: "Hey, Girl 1, using that word like that is appropriation."
Girl 1: "What do you mean? Of course not!"
Girl 2: "But you don't even speak Japanese..."
By It'sme33