As a self-proclaimed linguistic snob or commentator (depending on which end of my observations you land), I make it my business to keep track of how people are speaking. In the two years or so, I've noticed an increasing trend of using the phrase "I'm good." in response to questions like, "Would you like a seat?" or "How are you doing?" or "Can I get you a drink?" In all of these cases, the "I'm good" means "No, thanks" or "I'm ok" - and an implied, "no thanks." And while this phrase, "I'm good," seems to be harmless enough, it appears to me to be doing some damage.
Take, for example, the person who is trying on a pair of jeans and while assessing whether they define her ass well, emits "No, I'm good. I'm good. I'm good!" while pulling them off of her body violently. Or the person whose being coerced into signing a contract he doesn't want to sign, responding with "I'm good. I'm good!" These scenarios happen and they are examples of a problem with our new catchphrase.
"I'm good." isn't a catchphrase.
"I'm good." means "I'm fine." and "I'm ok." and "I'm sated." It DOESN'T mean "Fuck these jeans, they don't make my butt look good." or "I don't want to sign this contract because I'm getting screwed out of $1000." No, it doesn't mean that. As much as we try to force the round "I'm good" into the square "No way, Jose" hole, we are hurting our chances at communication. Using "I'm good" as a negative ("No, I don't want any" vs. "I'm fine.") is confusing and unclear at best and bastardizes one of the most simple of words in our language.
So, if you're using "I'm good" to mean "No thanks," to that, I say, "Nope, I'm good."