What I love is that these evolutions are normal. Each word is just the current rendition of something always in flux. “Darn!” someone says. But English speakers just two centuries ago wouldn’t have recognized it as a real word. It started with people exclaiming, “By the eternal God!” That, like many frequently used expressions, shortened: to “By the eternal!” Then, just as some people voiced “learn” as “larn,” many said, “By the ‘tarnal,” and then just “tarnal” as a stand-alone adjective. Mid-19th-century cartoons and commentary are replete with it.
One said “tarnal” in the same situations in which you might also at the time have said “Damnation!” Thus, people started exclaiming “Tarnation!” And since there was also “damn,” it felt natural to figure there was both “tarnation” and “tarn” or, sounding even more like “damn,” “darn,” emerging from a veritable buffet of changes.
These progressions, particularly the shortenings, can be especially awesome. I’m reading a novel in which one character, a maid, often says “yes’m” to the woman she works for. The “’m” began as two whole words, “mea domina,” which meant “my lady” or “my mistress” in Latin. That shortened to the French “madame,” which was passed on to English and shortened further to “ma’am,” and then the mere hiccup “’m” in “yes’m.”
This kind of thing is all over the place in just about anything anyone says anywhere on Earth. It’s why I’m not surprised that “brother,” a straightforward term for a sibling that might have stayed the same forever, has now come to mean “Oh, come on!” Not the bluntly exasperated “Oh, brother!” which is now rather antique, but something more layered, the laconic “bruh,” which expresses the ways in which life can saddle us with a lot. Someone says, “My mother wants me to tile her bathroom.” His friend shakes his head and grumbles, “Bruh …”
To me, these things are all examples of the inevitably approximate fit between words and expressions and what they refer to. To wit, it’s odd when words’ pronunciations and meanings don’t change, not when they do.