“Hey, you idiot, what’re you doing with that scythe? Man, you’re lucky you only got a little cut; you could have taken someone’s head off – or your own, which probably wouldn’t make a difference!”
Ooo… a scathing rebuke for one who escaped relatively unscathed from a misadventure.
Now, how is that? If the rebuke is scathing, then the person is scathed, right? Funny, though… the past participle adjective keeps its literal sense even when used consciously as a metaphor, while the present participle adjective is entirely figurative and refers specifically to use of language (speech or writing). We can see these patterns in the collocations: relatively, escape(ed), and emerge(ed) are the most common words coming just before unscathed, while scathing is most often preceded by wrote and issued and followed by report, critique, criticism, review, and letter.
Scathe was first of all a noun, meaning “harm” or “damage” (or, in the misty past, also someone who inflicted same). From there it easily became a verb (as so many nouns have, and yet despite cries of doom and agony from prescriptivist voices our language not only emerges unscathed from these conversions but thrives on them). The verb, in more recent centuries, took on a frequently more specific sense of scorching, blasting, burning, etc.; one may be tempted to imagine some phonaesthetic inclination affecting this, but although scorch and scald have the same onset and flame the same vowel, we must remember that bathe seems to have no such overtones, nor the fairly similar save either, and certainly not, say, lathe. The heat anagram of the rhyme portion of this word also probably had no formative influence on it, though it adds to the flavour now.
But scathing does lend itself to emphatic use, with the extensible lead-in /s/ and the opening-closing arc of the vowel /eI/ with an equally extensible and vibrating voiced fricative after it. Unscathed, for its part, gives a little echo effect when preceded by escaped.
And how does scathe get mixed up in all this? Well, it may be that he-cats (and any she-cat) have a decent supply of lives, but people less so, and even one who cheats death may still have to pick up the dice he cast.