It is when an unattractive girl/woman becomes attractive to you because you're very desperate or horny. You live with an ugly girl for a long time and all of sudden she starts becoming attractive to your eyes due to the fact that you have not it in a long ass time.
Expression inspired from "How I Met Your Mother" by Barney Stinson: "Evey girl has a mermaid clock
Some guy: Dude check out that ugly girl!
horny friend: Damn dawg shes looking fine, Im ready to dip it in.
Some guy: dawg you're disgusting...it's prolly cause you're being affected by this girl's mermaid clock.