Big, bigger, biggest. Small, smaller, smallest. That's how we learned it in school - the degrees of comparison. But then there were adjectives where adding "-er" and "-est" didn't work, and you had to use "more" and "most": curious, more curious, most curious. Alice, of Wonderland fame, grew "curiouser and curiouser" for humorous effect.
Going in the opposite direction, Alice would have been less curious and least curious. But when it comes to "less big" and "least big" - or "less small" and "least small" - things get confusing. What is "least small"? Is it the smallest, or is it the biggest among the small? You can't use "less" and "least" with adjectives signifying size if you want your language to be the least bit comprehensible. That's more or less all about "less" and "more."
Or is it? What about "lesser," the comparative of "less"? How can "less" have a comparative if it is already the comparative of "little" (little, less, least)? Yet, "lesser" has survived, notably in the phrase "the lesser of two evils."
Take "old, older, oldest." Have you ever heard it said: "He wasn't exactly an old gentleman, just somebody older"? This, paradoxically means that "older," a step beyond "old," is not older at all, but actually younger. On the other hand, we wouldn't say: "He wasn't a young guy, just somebody younger." Curious, isn't it?
Then there are adjectives of which we use only the superlative. We may have come across the oddest thing or experienced the tenderest of feelings, but never rated something odder than something else or certain feelings as being tenderer than others. T.S. Eliot designated April as the cruelest month, but he didn't say it is crueler than the others. (Leave it to the poets.)
Others may think of winter months as the cruelest - the worst, perhaps even the most worst. You can't say "most worst"? Then what about the "most unkindest cut"? What can we say? The poets again. Shakespeare this time.