What is a gay jam tart?

An old Cockney slang term, used to describe a sweetheart, as in sweet tart, in the rhyming slang of the time. It can also denote a wife, mistress, or prostitute.

Definition

More commonly known today as someone who is weak, or unreliable. A person who will back out, or avoid a confrontation or even commitment. Not a complimentary term, when applied to a person.