Similar to a standard simp, a fimp is a female that would continue to go after a guy, even if he doesn't bring much to the table or he has already cheated on her before.
Ashley: You heard about Rachel? She's always been going after this guy even after he had already cheated on her several times before.
Christina: Yeah. She's such a fimp.
(N) a mysterious person who shows up on your facebook friends list and you have no idea where they came from or who they are; are known as 'fimples'. Removing said person is known as "popping a fimple".