TV presenter Caroline Flack allegedly used the moniker to keep her relationship with the royal away from scrutiny.
"Caroline realises that Clarence House would frown upon her gossiping about her relationship, so she uses the codeword among her close circle of pals," the Sun quoted a friend as saying.
"She doesn't want to jeopardise what she has with Harry - and he has really taken to his new pet name," the source added.
The 28-year-old, who was said to be "besotted" with the 24-year-old prince, even posted a Facebook update, which read: "Caroline loves jam... x."