I dips me lid to that response but possibly “worst case” is a more colloquial equivalent?
Deriving from the original Latin, “pessimal” should mean “worst” - it is its own intensive, so to speak.
It seems the response time on this result, 13 years, is less than optimal. Perhaps we’re slipping.
Penultimate comes close…
That would be almost the worst possible choice. 
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I know the thread is old but can I ask for some opinions on whether or not a “Not X” is the same as the “antonym of X” as had been claimed early in the thread. I don’t think so but am happy to be educated.
If there was a word that meant the highest 0.01% the antonym would not be a word that meant all the rest (the not the top 0.01%, i.e. the lower 99.99%) but a word that meant the lowest 0.01%, yes?
Are there individual words in the statistical lexicon that differentiate between the positive and negative outliers?
I don’t think there’s any objective answer to this, “antonym” and “opposite” don’t have a precise logical definition. There are several similar words that have alternate antonyms, depending whether the emphasis is on the presence of absence of the specific attribute, or on possession of the opposite specific attribute.
local vs widespread / distant
assist vs abandon / impede
unique vs unexceptional / universal
You can also get a situation with three adjectives in opposition to each other. For instance, a color can be “light”, “dark”, or “bright”. Light blue and dark blue are in some sense opposites, but then, either one of them can also be in some sense opposite to bright blue.
It’s also not really an antonym; I see it as a minor modifier to the term. Something could be just a hair off and be “sub-optimal.” That seems to be the usual usage - “not quite as good as it could be” but nothing like “completely non-optimal.”
How much of each of those examples Riemann are actually functions of the fluidity in the meanings of individual words dependent upon specific context?
For example the antonym of “local” is very clearly “widespread” in the context of the following"
vs is clearly “distant” in this context
IMO you pretty well need an “anti” or “ante” in whatever word to describe the true polar opposite (to the degree there’s only one pole of opposition, ref Chronos).
The problem with “optimal” is that it’s a special sort of word for a special sort of situation. The sort of situation akin to temperature wherein there’s a clearly defined boundary in one direction (e.g. absolute zero), yet no boundary in the other.
“Such-and-such is *the *mathematically optimal solution to problem this-or-that.” OK. Sure. I’ll buy that.
Now what’s the *worst *conceivable solution to the same problem? I don’t know for sure, but whatever “worst” you might propose I can top it by adding more useless work to it. And that’s just in the class of solutions that (eventually) give the right answer. Considering the universe of non-terminating or wrong-answer-giving solutions and we’re looking at a set far beyond the size and scope of the real physical Universe.
The OP is a trick question because “optimal” is like the temperature or the positive integers. Bounded on one end and unbounded on the other.
But that same problem doesn’t stop us from having a word and concept for “hot”, nor did it stop the concept of “cold” before we knew of an absolute minimum temperature. We can still say that one thing is hotter or colder than another, just as we can say that one solution is more optimal or pessimal than another, and we can say that something is very hot or cold, just as we can say that something is very optimal or pessimal.
Wouldn’t the opposite of “optimal efficiency” would be “optimal inefficiency”?
How about “least” or “poorest” efficiency?
How about “Worst case”?
Catastrophal?
2016?
Yes - I was trying to find examples where the antonym was context-dependent but not for the trivial reason that the primary word can have multiple meanings.
i.e.
light - heavy
light - dark
…is not particularly interesting, that’s just alternate meanings of light.
Whereas in your examples, “local” means (exactly?) the same thing in both sentences, but the context indicates that the relevant antonym is different.
I say “sub-optimal” a lot, but usually I really mean “Not as good as it could have been.”, rather than “The worst! So sad!”.
Except that sometimes a thing is pretty crappy, and the while I might say “sub-optimal”, it’s really an understatement.
It depends entirely on how much irony is used, which is why it’s such a fantastic word. One might say, for example, that a burned-out husk of a factory has a sub-optimal production rate.
“Least sub-optimal” is also an excellent alternative to “optimal” when one wants to convey that while none of the choices are very good, one is better than the others.
Well, yes. We can use optimal and pessimal as directionals. Like north and south or hot or cold. Which really means more northerly, more southerly, hotter, colder, more efficient or less efficient than some implicit current state: typically the speaker’s location or the status quo situation.
To assert that something “is optimal” is another class of statement. It’s a statement of absolutes, not of relatives vs. some other identified state, or vs. an assumed middling-valued norm or status quo.
“It’s hot today” and “the process is working optimally today” are two very different things. The best equivalent word to “hot” on the optimality spectrum might be “efficient”. IOW, “It’s hot today” and “the process is working efficiently today” are parallel constructions. Where “cold” and “inefficient” would stand in the same relationship going in the other direction.
The unqualified “optimal”, like “unique”, or “absolute zero” is a different category of concept. At least strictly speaking.
Many businesses talk of optimizing this or that when they don’t mean it in the strict logical / mathematical sense. Rather they mean “make it reasonably close to ideal for the fat part of the envelope given the vagaries and unrepeatability and incompetence of the rest of our systems and our customers.” That “rule-of-thumb optimality” is a lot more like “hotter” or “colder”. It’s also business goofs abusing technical terminology like mad. Not a habit I choose to support. YMMV.
I use the term “suboptimal” when I make a mistake in a board game as in “that move was suboptimal”. It leaks into non game situations when talking with fellow gamers. Though it usually does not mean the worst possible (move), just not the best.
Brian