I was just thinking that sometimes it doesn't matter what you say, the meaning will still get across. I was going to mention the "55% of communication is body language/facial expression" factoid, but it seems that's been debunked. Someone should probably tell my Dad, who I'm pretty sure firmly believes it. Anyway, my point remains: in certain contexts, words are malleable to whatever you're trying to say - or whatever the person listening to you wants to hear. It's interesting.
The example that made me think this in the first place - how do you agree to do something, or express enthusiasm for it? Two things you could definitely say are "I'm up for that." and "I'm down for that." Nobody is really paying attention to your exact wording when you say things like this, they just hear "Yes", but it struck me that "up" and "down" are exact antonyms, and yet they don't change the meaning of the phrase at all. You could mumble the phrase and it wouldn't make a difference, since the listener could probably still deduce your meaning from your tone of voice.
Another antonym pair is a little different. Saying, for example, "He's in for a shock" comes across as the subject having something coming unavoidably at him. Then you have, say, "She's out for a punishment" and it seems like she's actively seeking this punishment - but they're still not opposites. Because phrases are weird. (Then again, they're not all that bad in English - just wait until I show you some Irish ones, then you'll know about weird phrases that really, really don't translate well literally.)
Another example: there is a ridiculous amount of words for what we usually call "cool" (neat, wicked, awesome, hip, rad, savvy, trendy, all right, hot). I know most of those words sound ridiculous now (the word "hip" is a joke in itself) but they are or were all used for that general concept of coolness. And "hot" and "cool" are essentially opposites, yet they're used in practically the same way. This one comes mostly from slang, I think. Still.
And beautiful, but in a sexy way: hot, fit, buff, peng.
So I'm going to arbitrarily declare that bombastic (adj., full of hot air, high-sounding with little meaning) is equivalent in meaning to attractive, and boom! The language changes. Wait, what? It doesn't? I have to wait for it to catch on? But how do I predict what's likely to catch on and what isn't?
Language is weird.
And then you have the fact that "literally" now has two meanings in the Webster and Cambridge dictionaries, and they are complete opposites. Go figure.