When I first started working on Robert Lowth’s letters, and in particular on the letters he had written to his wife Mary, I happened to tell a colleague about them. The colleague was surprised to find that Lowth’s private name for Mary was Molly, as this seemed very unusual to him. But the Lowths also had a daughter called Molly, or Mary in full, and in the letters he frequently refers to her as “ye. [the] dear little Molly”, and even as “ye. little Mollykin” and, using another endearment device, as “the dear Toms [Lowth’s eldest son] & Mollys”.
So it was interesting to read that John Wesley, too, addressed his wife Mary as Molly, as Annemiek Korf shows elsewhere in this blog. So this form of endearment for the name Mary turns out not to be so uncommon after all. Another Mary-called-Molly is Molly Pitcher, whose full name was Mary Ludwig Hays McCauley (1754-1832). A fascinating woman, I would say, as she is described as a camp follower but also, as this image illustrates, as a soldier!
Any other Molly’s, heroic or otherwise?
Late Modern English letters 
It is actually quite common to use Molly as a nickname for Mary. It can easily be explained in linguistic terms, when looking from the name-truncation-perspective.
When I wrote my Bachelor thesis on name truncation in 2011 I went to England for a week and asked people on the streets about their own and friends’ names and how these names got shortened. I did similar research in America, only then through Facebook. I collected 300 name pairs in America and 324 name pairs in Britain.
The change from Mary to Molly is a progressive one, which means that there are several smaller steps that happen in the process. These steps however, can happen almost simultaneously, but still have to be seen as separate steps. Normally, the first step would be adding the diminutive /-y/. This is a very common thing to do in name truncation: in 32% of all the collected names from the USA a /-y/-diminutive was added, and in 24.7% of the British data. However, the name Mary already ends in /-y/, so this is not necessary anymore.
The second step, then, is changing /r/ into /l/. The /r/ and the /l/ are two very similar letters, as is shown in any phonetic chart (eg. http://tktandreasalaso.files.wordpress.com/2011/07/imagen2bblog8.gif?w=589&h=377). As you can see on the chart, the /r/ and the /l/ are immediately next to each other, so the change from one to the other is only a small one. Step two means: Mary becomes Mally.
The third step is easy as well. Because it is phonetically easier to say an /o/-sound before and /l/-sound than it is to say an /a/-sound, /a/ gets changed into /o/. Change of vowel is also very common in name truncation: it happened in about 15% of my data. So, step three means: Mally becomes Molly.
A potential next step is rhyme truncation: which means that Molly would become Polly. Other progressive rhyme truncations are for example Richard – Dick, and Margaret to Peg or Peggy. About this last name truncation there exists a lovely little poem by an anonymous poet:
“In search from A to Z they passed,
And “Marguerita” chose at last;
But thought it sound far more sweet,
To call the baby “Marguerite”;
When grandma saw the little pet,
She called her “darling Margaret”;
Next uncle Jack and cousin Aggie,
Sent cup and spoon to “little Maggie”;
And grandpa the right must beg,
To call the lassie “bonnie Meg”;
From “Marguerita” down to “Meg”.
And now she’s simply “little Peg”.”
If anyone is interested in reading more about modern and historic name truncation I can recommend my own research, so just let me know!
Reference:
“A Survey into British and American Name Truncation: How Richard Became a Dick, and William a Willy,” by Annemiek Korf and Joyce J. Knol
I’m impressed!
Karlijn Navest in her article on letter-writing formulas in Reynolds’ correspondence writes that Reynolds addressed his favourite niece Theophila as “Offy”: does your account also explain how this nickname would have come about? And the next step of Mary -> Molly -> Polly is taken by Reynolds when he addressed Mary Palmer not as “Molly” but as “Polly”.
I imagine ‘Offy’ is from the ‘oph’ portion of ‘Theophilia’?
Didn’t William become a Billy?