Simple Definition:

A simp is a slang term used to describe someone, typically a man, who is overly attentive, submissive, or fawning [displaying exaggerated flattery or affection] towards someone they are romantically interested in, often to the point of being considered desperate. For example, if a man consistently spends all his money on a woman who isn’t interested in him, his friends may call him a simp.

Very Simple Definition:

A simp is a term used to describe someone, usually a man, who is attracted to someone, and will do anything to get their attention or approval. This term can be seen as derogatory or insulting.

Example: John is a simp for Jane because he always does whatever she asks and buys her gifts.