Sempai is a misspelled form of the widely known Japanese title, people bestow on the senior of the group.
Depending on the context, sempai may mean mentor or senior or even master, in certain situations.
In English we use it to give voice to our affectionate feelings, as well as respect in a humorous way.
The term had been in use in Japan for centuries, and though they are reserved for people of stature and respect, it is solely utilized for mentors, so a teacher in a high school doesn’t qualify for being called “sempai” by all their students… perhaps by a few, that look up on them.
“Sempai” got more and more embedded in the English lexicon after the 80’s, when interest in Japanese culture rose, with karate clubs and judo dojos opening across the United States.
A growing base for the otaku group also resulted in more familiarity associated with “sempai”.
The reoccurring theme of feeling an overflowing amount of affection toward “sempai” resulted in it being often mentioned on sites, like Tumblr.
In 2012, the single serving site “Has Senpai Noticed Me Yet?” was launched, which in turn led to the creation of the “I hope sempai will notice me” meme, which had been circulating all across the worldwide web during the 2010’s.
Image macros and other various forms of memes had been created, paying homage to the theme on all platforms from Facebook meme groups to YouTube clips, like PewDiePie’s “NOTICE ME SENPAI!!!!” video.