"Euphemism" is hardly the right word. "Dysphemism" is more like it.

Concerning the minstrel song, at least: 19th century databases show that "hog-eyed" was a southern (American) term for "having small or squinting eyes."

One newspaper contrasts it with "buck-eyed" (modern "bug-eyed").

The simplest explanation is that the original "Hog-Eyed Man" in both the minstrel song and the popular fiddle tune, was simply a squint-eyed or small-eyed fellow.

As a kind of barge in use in San Francisco Bay, "hogeye" must have been a pretty local term. It doesn't appear in either the American "Century Dictionary" of 1889 or the "Oxford English Dictionary" of any date.

The original squinty guy must have been reinterpreted by those familiar with the sexual sense of "hogeye."

Few of them could have been thinking of California barges, in either case.