When a guy has been hanging out with, or simply in the presence of a girl that is just a friend for long enough that she ends up becoming sexually desirable.
Based on the idea of sex-starved sailors who would see manatees frequently and eventually they began to see them as mermaids.
It was also used in the TV show How I Met Your Mother.
Guy 1: "So you're going to be rooming with a chick next year? watch out for the mermaid effect, if you hang out with her long enough, you know you'll end up wanting to hit that."
Guy 2: "Nah man, she isn't hot and totally isn't my type."
Guy 1: "I give it three months."
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