Fairly common endearment.

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Note added at 5 mins (2011-02-27 00:05:17 GMT)
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Or Mumsy.
Another way of saying mommy/mummy. Can be more affectionate or used when the child wants something, usually money. Or a way to describe something ...
www.urbandictionary.com/define.php?term=mumsy

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Note added at 14 mins (2011-02-27 00:14:24 GMT)
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Or just Mommy.

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Note added at 12 hrs (2011-02-27 12:56:53 GMT)
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Here are some examples of recent usage:
Thank you, mumsy (and father). Life is indeed serious business, and should be approached as such.
www.huffingtonpost.com/jason-kitchen/to-my-dearest-mumsy-th...
Our Mumsy and Nana, you accomplished so much in your life. ... Our Nana and Mumsy, we will never forget you; we can't! ...
www.joannebayvel.wordpress.com/2010/07/18/our-precious-na..
Who do you call mumsy woman?
www.answers.yahoo.com/question/index?qid=20080702025533AAex...
www.spunkymumsy.blogspot.com/2008/08/and-so-lion-fell-in-..


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Note added at 13 hrs (2011-02-27 13:18:58 GMT)
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And here's Momsy:
i just lost mom, i would call her momsy.
www.hubpages.com/hub/Thank-you-mother--mom--ma--mommy--moms...
This is Tyler Russell Hill's proud mom...whom he affectionately referred to as:MOMSY
www.disboards.com/showthread.php?p=27564571
They love the pool, so that's almost a must, and momsy wants a balcony !!
www.geoexpat.com/forum/61/thread165321.html
Had dinner at momsy's house before she heads off to the airport.
www.angbuhayniela.blogspot.com/2009_01_01_archive.html -