ffk is also known as 'fong fei kei' in cantonese
Is to break a promise, a deal or did not show up for an appointment
Orgin : Making a promise with a friend is similar to railing out a kite. If you keep your promise, means you rail back your kite hence keeping your friendship. To break a promise, is like cutting the line of the kite and it will fly away similar to loosing a friend.