Which is he more likely to do when about to dive, beat his breast or incline his upper body toward the water? I suppose he could beat his breast to show how gorilla-like he is, but that seems to be a modern gesture more closely associated with pro wrestlers than with cabin boys.

Moreover, if "bent his breast" is just a mondegreen, what's Goldsmith doing with it? Conceivably he picked up the phrase from a ballad; but if it made such transparent sense to him that he could use it in a literary tale (in an age notorious for prescribed and approved diction), there's really no reason to assume that the idiom was ever just the result of mishearing.

If it made sense to Goldsmith - and to some others here - it could just as easily have made sense to the ballad writer.

Finally, if anything is a mishearing in these songs, my money is on "beat his breast." This thread shows that "bent his breast" can sound not just unfamiliar but incomprehensible and unbelievable to many people. So it would seem more plausible that an original"bent" was altered to "beat" than the other way around.

No?