In the 70's the term 'Rap' had a different meaning than is popularly used today. It basically meant 'to talk'.
Mettings and group idea exchanges were called 'Rap Sessions'.
When leaving a friend you would say 'I'll rap with you later'.
I guess the term as it is used today came from the MC 'rapping' with the audience.
Using the term today like it was used back then sounds a bit cheesy but maybe it'll strike a cord (hehe) with that particular generation.