To understand the definition of an anti-simp, one must understand the definition of a simp, make it fit to all genders and then take the opposite.
An anti-simp (noun) is someone who does simp for someone without a rational reason, and then does the opposite of simping to show they don't simp.
Anti-Simps would change their opinions, like simps, to oppose people. So they will always be on a negative standing with whom they anti-simp (verb).
1st conversation
Jack: Want to go eat pizza?
Anti-Simp: No, I hate pizza.
2nd conversation:
John: I don't like pizza.
Anti-Simp: I love pizza.