I had a great uncle who said a 'flip-flop' was an old slang word for a radio. Has anyone else ever heard this?

I never heard of that. I’m 67.

It is a lie.

The only thing that I can think of that connects radio to flip-flop is those alarm clock radio’s that have flipping digits to show the time.

I think I will ask my mother if SHE ever heard my Great Uncle make this claim.

It’s possible, depending on which circles one might have frequented.
Merriam-Webster’s Collegiate Dictionary, 11th ed., says:
“flip-flop…..3 : a usu. electronic device or a circuit (as in a computer) capable of assuming either of two stable states…”
Now one might assume, possibly, that these “two stable states” might be SEND and RECEIVE or something other…

I’m 71 and I’ve never heard of this. There are however, many slang terms that are local to a specific area or even town. I wouldn’t write it off just because I haven’t heard it.

Where I live, “Flip Flops” are a form of footwear. Usually, a sole and a strap that goes between the big toe, and the other toe.

They are very common among “beach” cultures. I prefer sandals (no between the toe stuff,) and wear them often in the Charleston heat….

There is an answer on Quora that connects radio with flipflops. I guess it was a slang used in radio communication.

A “flip-flop” on Citizens Band (CB) radio is a trucker slang for doing a U-turn or doing a round trip. (up and back on the road).