There are a couple of other cases. Dog was once (1845, UK) slang for sausage, due to fears of dog meat. Then a hot sausage was a hot dog, and then hot dog was a sausage, and finally just dog was slang for a sausage again, with the sinister implication removed. I also found pepper, from Latin (from Sanskrit?), leading to pep, the thing a pepper does to you, leading back to pepper (a kind of pre-game warm-up in baseball designed to give players pep).

You might object that those are really back-and-forth, but so is the claim being made here about slop. (I kind of doubt the claim though.)