||The Oxford Names Companion, OUP
"Scottish Forenames" - Donald Whyte, FGH, FSG
Private communications [AB, JMcF, MG, ML, MC2]
English. Although most frequently a short form of Jennifer, Jenny was in use in the Middle Ages as a pet name for Jean/Jane.
In Scotland Jessie is the more usual pet name or synonym for Janet [a derivative of Jane, itself a synonym for Jean], but several correspondents [AB, JMcF, MG, etc] record occurrences in Scotland of a person known as Jennie or Jenny having been baptised as Janet.
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.