The only real way to shorten this would be "Merce." Which sounds like "murse," which stands for "man-purse," sooo ... yeah.
Famous people named Mercy:
Actresses Mercy Gentry, Mercy Johnson, and Mercy Fiennes-Tiffin; figure in the Salem witch trials Mercy Lewis.
There is a character named Mercy Wood in the novel The Witch of Blackbird Pond, which will almost certainly be required reading at some point during your little Mercy's schooling.
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