A characteristic of a person: Someone who is likely to change their mind about having sex often, and sometimes without warning. This is a sub-category of the word 'fickle', meaning to change one's mind frequently.
Guy: "Fancy a fuck?"
Guy grabs her tit Girl kicks him off: "I don't want to have sex!"
Guy: "You just said you wanted to!"
Girl: "I do!"
Guy: "You're so fuckle!"