Plum-Plum is the phenomenon of inevitable miscommunication between two ore more persons.
This is due to the fact we can never be sure that two people understand a particular word in the same way. Hence, any word used will inevitably be understood differently by two different people. Furthermore, each person potentially uses and understands the same word in various ways at different times.
This inevitable miscommunication, is grosso modo, the plum-plum. *
* If you think you understood all this, then you have just experienced plum-plum for the first time.