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3.
When a dog or cat is sitting and they raise their hind legs so that their anus is in direct contact with the carpet and then use their front legs to drag their ass around. Sometimes it leaves a stain.
Fido just merled a nasty stripe on your mom's new carpet.

The cat is merling at least a dozen times a day, it may be time to hit it with a shovel.
by mcgillicutty April 04, 2003
 
1.
Half man, half girl. Can drink as much as a college boy, but still has female sex appeal. The most notorious MERL, rumoured to be named 'Noor', attends the University of Colorado at Boulder and can be spotted at the bars on a daily basis on the prowl. Though there is no actual documentation of this MERL, many have claimed to have spotted her on several occasions. Use caution in the presence of a MERL.
"God I love being half man, half girl"
"MERL, you probably shouldn't say that in public anymore"

"MERL you are the coolest 'guy' in the house"

by cupcake16 January 21, 2009
 
2.
A term used as an insult.
Somewhat like the word jerk.

Guy: "Hey, you wanna go get laid?"
Girl: "No, you're such a merl!"
by Erika Elite Tx March 16, 2008
 
4.
The first letters of the fabulous four girls from Indiana University! Yay MERL and Hoosiers too!
No group can compare to the wonderful group or MERL.
by Lauren (the L in MERL) October 17, 2003