English and Scottish, mainly a pet name, originally for Jean (ONC).
In Scotland, Jessie is used synonymously for Janet for which it is also a pet name. It is, however, a given name in its own right. The Gaelic equivalent is Teasag.
Some recent research [by AB] has shown that Jessie has been gaining in favour since its introduction in the mid-18th century. In the 1770s the ratio of Janet to Jessie baptisms in the I.G.I. was over 400:1. A century later the two names reached parity in the records. We would be interested to hear if anyone has a theory of why this change came about.
The 1841 Census of Aberdeenshire has a number of significantly frequent abbreviations and spelling variations. These are listed here, although most will be found using Soundex searching. However, there is one occurrence of Gessie which might not be found that way.
Janet is derived from Jane [q.v.] but the relationship between Jessie and Jane is unclear. A correspondent [AB] records a case of a child baptized Jane who, in later years, was known and recorded as Jessie. The ONC suggests that Jessie was originally a Scottish pet name for Jean. In as much as Jean is synonymous with Jane in Scotland there is some connection but this does not seem to be supported by most Scots researching their family histories.
A second correspondent [RA] reports an aunt, baptised Jane, who was always known by the pet name Jessie despite having a sister who had actually been baptised as such!
It is therefore possible that the names could, on occasion, be interchangeable. However, as Jane is only very loosely linked to Jessie, we have classed this link as a Lesser Synonym.
The rare use of Jessie as a male name appears to be a re-spelling of the Biblical name Jesse [q.v. for additional notes] but there is no clear connection between the male and female versions.
Jessie as a pet name for Jessica is mainly an English usage. Jessica does not occur in the 1841 Aberdeenshire Census and is believed to be very rare in Scotland.
A Lanarkshire correspondent [MC2] has a family member, baptised as Jenny (not Jennifer), who was known by the pet name Jessie. She had been named after an older relative Janet, which had already been used in the family and might explain the unusual use of Jessie as a pet name for Jenny.