Being a SIMP means having a too much sympathy to some other person with expecting something in exchange, there are also simps who does not expect anything and any contact with girls is enough to satisfy them.
Simping is having too much sympathy to someone, mostly to girls, even simping to 3d girls exists
Being too much attached to some character, adoring that character is also SIMPING. Defending a 3d girl when defense is no needed or being jelaous about 3d character is also Simping.
If someone is a SIMP in this means that person adoring them and have too much sympathy for female characters or real life girls.
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I've tried to explain this in most simple way as i could