English modified form of Molly, both widely-used pet names for Mary [q.v.]. Polly has also been used as a pet name for Paula. We also know of one use as a pet name for Pamela but this may not be a general usage.
A correspondent [MB] sent us a transcription of a letter dated 1796 in North Carolina, USA. In the letter a child named Mary is frequently referred to as "Polly".
Polly was derived from Molly at the same time as Peggy developed from Meggy (pet name for Margaret) and other "M-" to "P-" conversions. The reason for this consonant shift is unclear.
A Scottish correspondent [MC2] reports cases where either Molly or Polly was used as a pet name for Mary, but was sometimes developed to Dolly, although this is very unusual.
Another correspondent [EK3] reports a relative whose name was Mary, pet name Polly; she didn't like her given name so told everyone her 'real' name was Pauline. Another [KM] has confirmed this usage. This is a natural reduction so we have included it here.