An oxymoron is a contradiction in terms, like “industrial park” or “holy war” or “jumbo shrimp.” It's sometimes used derisively, for example suggesting that “business ethics” or “military intelligence” are oxymorons. Oxymoron as a contradiction in terms didn't show up until 1902 when someone who worked hard at being lazy was said to be a living oxymoron. Before that oxymoron was a technical term describing a rhetorical device. The word first showed up as an English word in 1657 in a book called The mysterie of rhetorique unvail'd where the editor, John Smith, defined oxymoron as “subtly foolish.” You might think that being subtly foolish is an oxymoron in itself, yet this definition goes a long way to uncover the etymology of oxymoron . The word has Greek roots but didn't exist in classical Greek. Instead it was constructed from parts that did exist way back then.
The first half of the word oxy - means “sharp.” This meaning is contained also in oxygen , the name of the element we need to breathe to live. During the late 1700s when chemistry was not quite as sophisticated as it is today it was thought that acid was produced from oxygen. Since acid has a sharpness to it, the stuff that creates acid was called “sharp maker.” Or, if you translate that into fancy scientific Latin and Greek, oxy-gen ; since gen means “make” or “generate.” The second half of oxymoron is - moron and yes, it really does mean “stupid” in Greek. So that first English citation defining oxymoron as “subtly foolish” is a literal translation, oxymoron really does mean sharp and stupid at the same time.
And if that's the case, then the word oxymoron is itself an oxymoron.