Clark is from a small, backwoods town in Southern U.S.A. So, even though he lives in Metropolis - the 'big city’ of big cities - now, his roots are still just that: roots. Things so deeply embedded in the essence of his soul that no matter how much of a city boy he becomes, will stick with him until his death.
Some of those things may entail, but are not limited to: saying out of date phrases, idioms that confuse everyone that's not from a backwoods southern town, being very particular about his manners, and being very particular about his language. These little quirks and details that may bypass the average eye are all too obvious to his lover.
Truthfully, it's kind of hard to miss when, after waking you up in the sweetest, most ‘Clark Kent’ way ever, he spills some coffee across the counter and his immediate reaction is to whisper “Aw, shucks.” Noticing the confused expression on your face, he gasps, covering his mouth with his hand. “Gee, I'm sorry. I didn’t mean to use such harsh language.”
It's almost funny - in what world is “Shucks” considered harsh language? He does seem to be at least a little upset, though, because he's looking at you like a kicked puppy. And, because he's Clark Kent, it's more of a ‘you just ran a puppy through a paper shredder’ look. You watch his expression sour as he remembers the coffee now spilled all over the counter. “Rats,” he mumbles, grabbing a few paper towels to start cleaning it up.