Originally it was more about not idolizing a girl only because of her gender. On the streaming site "Twitch" there are some girl streamers who show a bit of cleave and wear short dresses to get viewers. These viewers then create parasocial relationships with the streamer and basically idolizes them for being pretty. Because of this the viewers donate money to the streamer for different reasons but mostly to make her happy and get her attention for a few seconds.

So what happens is that people criticize these streamers for taking advantage of lonely people and not really bringing any content to the platform other than their body. So these kind of streams are basically girlfriend simulators and the poeple who watch them and support them with donations are so-called "simps".

Nowadays the word "simp" has turned into a slurr for anyone who shows any kind of affection to a person.