The commentary from a recent post alights on some very basic (but always interesting) historical linguistics topics.
Textbooks are always sure to provide a list of traditional semantic changes including amelioration and pejoration, and common examples such as knave churl hussy nice...etc. The following list gives some loose examples of amelioration but it's not exactly the type of elevating shift that is intended by the term.
bad sick fierce mad wicked ridiculous crazy insane terrific awesome
Rather than having shifted meaning by undergoing simple amelioration the first eight of these are a type of contranym (or autoantonym, or "janus word" or several other rarely necessary words that few dictionaries list). But those labels usually indicate a word like cleave or peruse that is equally likely to have contradictory meanings without consideration of style or register while the first eight words above typically have clear meanings based on the context. Consider the following pairs of uses:
She is a bad mother.
She's one bad mother!
Trying that stunt was ridiculous.
Dude that 360 nac-nac was ridiculous!
In formal speech the 2nd meaning would be very odd for each word. Like ridiculous in slang usage crazy and insane don't simply mean extremely good. They have a connotation of impressiveness and "hard-to-believability" (that was awkward).
Terrific and awesome are good examples of a complete semantic shift, having abandoned an earlier meaning. Awesome has the interesting tie to awful which has kept its negative meaning. We can trace awe back to OE ege and its cognate in ON agi: fear and terror.
Amelioration and elevation don't quite capture the force on these words. There is an inversion that isn't explained by simple lenition of the semantic force. In fact there is no lenition at all, but an equally forced reversal; there is no significant loss or gain of neutrality in these words.
The common theory that a word must go through polysemy in order to acquire new meanings is probably quite relevant here. For a word like terrific there was clearly a time when the more specific meaning "causing fear" generalized to include other emotions before moving over and settling on "worthy of praise." But what generalization and polysemy was there with bad? In some of these words like insane and mad there is that space where they are used to concurrently indicate positive attributes like bravery and daring while also indicating a lack of good judgment.
How about "that's sick"? Maybe something like "It's sick that anyone should be so talented. I'm jealous of your skill." Wicked? Perhaps an intention like "You must have connections to darker powers to be so successful." Both of these jocular in use. But there always a little truth in any good jest no?
And bad might follow right along with that but with a more basic sense of the fascination we have for those people who break rules and show no fear or remorse. In fact the OED suggests in its very recent draft addition the meaning of "dangerous, menacing, or imposing to a degree which inspires awe or admiration; impressively tough, uncompromising, or combative."
Considering all this we go back to a word like comprise, which when considered alongside a word like apprise is evidence that the mental representation of a word's semantics is much more broad than just to include the intention. Somewhere in that encyclopedic matrix of 'what this word means' is an awareness of what the word does not mean and a sense of the word's opposites and even complements. So when called up, a word like start brings with it a role of opposite of stop. That's pretty clear. What I find fascinating is that the understanding and use of a word like gather comprises not only disperse but also be gathered. And a word like learn travels with be taught and forget and teach in its pocket. These words must have such pockets. Otherwise it'd be hard to understand why we are always finding new meanings in there.