Polly.

Mary. The change of M for P in pet names is by no means rare; e.g.—

Margaret. Maggie or Meggy, becomes Peggie, and Pegg or Peg.

Martha. Matty becomes Patty.

Mary. Molly becomes Polly or Poll.

Here we see another change by no means unusual—that of r into l or ll. Similarly, Sarah becomes Sally; Dorothea, Dora, becomes Dolly; Harry, Hal.

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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.