A common use is as a greeting. For this, the body language needs to be firendly - smiling, raised arm etc - and the word needs to be said loudly and clearly. It is particularly effective if the word is held on (in other words, saying merrrrrrrrrrrrrr).
Another common use is just to drop it in moments that lack conversation, or to express boredom. Again, it works best to hold the word (merrrrrrrrrrrrrr), and to say it only through one half of your mouth as if muttering.
It can be used as a way of agreement. In these cases it should be short, clear and often preceeded by 'ah'.
The word comes from Buckinghamshire, UK and is the nonsesical muttering of a certain someone.
What the merr
Jackie, Mike, Caroline
Second Period Humanties II 2006
i hate life.... merrr
Lets Merr in the back of my Merrvan.
Used in conjunction with random interjections of statements describing obvious things about the person.
Ryan: MERR! MY NAME IS CRISTINA! MERMERMERMER! I ENJOY SEMIFREDDO ICECREAM! MERRR!
Aaron: MERR! I'M SARAH PALIN! I CAN SEE RUSSIA FROM MY HOUSE! MERMERMERMER!