It is a theory about a woman, no matter how ugly she may seem at first, will eventually look beautiful to you if you see her every day all day.
Nick : Hey! Dude Zara is so hot!
John : Are you crazy?! It's probably just the mermaid theory. you have her in every class.
Extra for How I Met Your Mother fan club
The Mermaid Theory is the theory that a man will eventually want to sleep with any woman after a period of time, no matter how initially repugnant. The theory is so called because sailors (according to Barney) would spend so long at sea that they would eventually come to view manatees as beautiful mermaids.
A mermaid will become unattractive (turn into a big ugly manatee) again by becoming pregnant. However, she will become a mermaid once again by breastfeeding.
