Until a couple years ago, I’d never even heard the term “unthaw,” but I can guess it’s meaning from looking at it. Un means not, so it means freeze or frozen, right? Well yeah! It does! except…
Most people who use the word actually mean “thaw” when they say unthaw, which is confusing and dumb. And also, it’s correct!
You see, almost every dictionary website out there lists both definitions as valid for this word, and Marriam-Webster only lists the second one. Apparently people have been saying unthaw to mean melt since the year 1598. Probably before that, but it’s first written use was at that time. It begs the question though, why are people saying something that sounds so dumb, when they could just chop off that first syllable and it’d make more sense? My guess could only be that people say it to convey that it’s more than just thawing. Like your car doesn’t need to thaw, it needs to UNthaw. Makes it feel more dramatic.
Either way, the important thing here is that people are saying it. People have been saying it for centuries, and they aren’t going to stop. Just because it sounds dumb or contradictory doesn’t make it not a word. It’s like “irregardless.” It’s a word because people are saying it, and it conveys the meaning that they mean for it to convey. It’s a word you might not like, but it’s still a word.
I see this all the time when people find out that whatever slang term they hate has been added to the dictionary. They complain that the English language is being destroyed and that words like that don’t belong in there, but think about it this way: Who reads the dictionary? If you’re fluent in the language, you’d only be reading the dictionary for words you don’t know, but someone who is just learning the language is going to almost constantly hear words they’ve never heard, and slang is chief among those things. If you said derp online, it’s probably better that someone be able to open up their Oxford dictionary and learn the meaning, rather than you needing to explain it to them.
You can complain all you want that English is changing and that it’s being destroyed when those things are added, but the thing is, the language has already changed. The dictionaries adding those words is just them reflecting that change. Also, a language growing and changing is a sign of a healthy language and society, and clinging to rules and trying to force everyone to speak a specific way is how you wind up with dead languages.