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1.
-A noise one makes instead of a sigh.

-A noise one makes that indicates apathy towards whatever is occurring at the moment.

-A noise one makes when one is bored or uninterested.

-It can also be used as a response to something when there is no answer for it, or if you do not know the answer to it.

-It could also show ones agreement with another person when one is not in the correct mindset to answer with a word or sentence.
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Ex 1

Gigi- How was your day dear?

Portia- mer mer mer

Ex 2.

Todd- That was good sushi.

Courtenay- mer mer mer
by PeytonCreauton December 08, 2008
 
2.
What is said when one is asked to recall a conversation or statement made by another, especially if the information was mundane or the person saying it was inconsequential.
Andrew: I'm so cool and you just think you are. Hahaha

Me: "Huh? Did you say something? All I heard was "mermermer".
by MauiMami December 05, 2010