-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.
Gigi- How was your day dear?
Portia- mer mer mer
Todd- That was good sushi.
Courtenay- mer mer mer
Me: "Huh? Did you say something? All I heard was "mermermer".