A phrase coined during the 1960s and 1970s trucker Citizen's Band (CB) culture. It literally means "I'll contact you on the return trip." If two long-haul truck drivers regularly make opposing trips, their path will eventually meet on each iteration of the trip. This term is virtually interchangeable with catch you on the flip side.
...we're gonna back on out of here. So, keep the bugs off your glass and the bears off your tail. We'll catch you on the flip flop. This here's the Rubber Duck on the side. We gone. Bye bye.