Let me be succinct.
OK, I'm "succinct." Now how do you say me?
I did a little poll about this on Twitter and the results were interesting. Of 416 respondents, 55% said they pronounce it like "sus-sinct" (or "suh-sinct"), and 40% said they pronounce it like "suck-sinct." But what was even more interesting was that many of the respondents, however they said it, were shocked that anyone would pronounce it the other way.
A few respondents also pointed out rather primly that we always say cc before i or e as "ks" (except in Italian loans like focaccia, Latin words like Staphylococci, and of course soccer), so, clearly, "sussinct" is not asseptable and anyone saying it that way must be doing it unsussessfully by assident.
But there are three problems with that line of reasoning.
First, the "sussinct" pronunciation is enshrined by such authorities as Merriam-Webster as one of two acceptable ways of saying the word.
Second, English loves weird little exceptions — or, more to the point, English-speaking people expect weird little exceptions (like the spellings of weird and people), so English just keeps getting them.
And third, there are other exceptions to the cc = "ks" rule.
The most prominent exception is flaccid, for which authoritative dictionaries (Merriam-Webster, Oxford, etc.) give "fla-sid" as the first pronunciation, before "flak-sid." Not that everyone agrees! Debate is so heated across the "flaxid"/"flassid" divide that Bryan Garner, in Garner's Modern English Usage, calls the word "skunked" and implies you're better off not saying it at all.
There are also other exceptions to the cc = "ks" rule that you may hear from time to time. In at least some circles, some people pronounce eccentricity without the "k" sound. Other words that have lost the "k" for some speakers include successful, accelerate, accessible, and accessory — and Merriam-Webster has even added a "k"-less pronunciation as optional for accessory, though not for the others.
OK, but why? Again, there are a few possible reasons, and probably all of them are in play here.
First, there's economy of effort. If we can get away with less physical exertion while speaking without becoming incomprehensible or being condemned as ignorant louts, we often will. A word like succinct requires quite a bit of exercise from your tongue: from "s" to "ks" to "nct," it alternates tongue-tip touches with rolls forward from the back, like a break dancer doing a caterpillar. Changing "ks" to "s" cuts the effort nearly in half. Even a word like flaccid or accessory is just that little bit easier without having to touch the back of the tongue to the roof of the mouth for a "k."
Economy of effort doesn't by itself account for the change, though, because there are many other words where we could drop the "k" but don't — vaccination, for instance, has no real risk of being mistaken for something else if it's said as "vassination," unlike a word such as succeed, which will sound like secede if it loses its "k." But it's easy to see why, when we're given the option of dropping the "k," many of us take it.
Second, there's analogy. Our pronunciation of a word is influenced by other words that it seems like, and we don't always keep an eye on the spelling or etymology. Flaccid, which seems to have been the first of these cc words to lose its "k" (authors were already inveighing against this "error" in the 1800s), is rather like acid and placid, especially if you don't always remember how many c's it has: you'll find it written as flacid in various texts from the 1700s to today. Similarly, accessory has more in common, sound-wise, with assess than with access because of its second-syllable stress, and neither of those words seems to have more in common than the other with accessory, meaning-wise. Analogy and resemblance may not have as much of an influence on succinct, but if you're used to hearing "flassid" and "assessory" you have some precedent for it.
Third, there's social pressure. If our friends do it, we often do it too. Yes, yes, if your friends all jumped off a cliff, you might think twice about it, but on the other hand you might assume they know something you don't. And when it comes to English pronunciation, where we have all learned that we will always be wrong at random times for no good reason, and where saying a word differently seldom leads to actual death or injury (except to our pride), if our friends are saying a word a particular way, we'll likely assume it must be right, even if (or especially if) it's weird and unexpected. That doesn't account for the start of a variant pronunciation, but it can certainly help it spread.
True, social pressure also accounts for why we keep saying a word the "correct" way, but it also sometimes helps the "correct" way to change. Take the suffix -ing for example. It's a little easier to say "in" than "ing" in many contexts, and by the early 1700s, the pronunciation of -ing was well established as "-in" even for high-status speakers such as poets and the nobility; saying saying as "say-ing" rather than "say-in" would have been an overpronunciation, uneducated, like saying "cup board" rather than "cubbard" for cupboard. But then something happened: some schoolteachers and people who wrote grammar books decided that this was lazy and incorrect and that -ing should be said the way it's spelled. And, faced with hearing it one way from their peers (or from the peers of the realm) and another way from their teachers, speakers went with whoever was most likely to punish them for saying it "wrong" — or they alternated depending on who they most needed to impress. So for the past couple of centuries, it has once again been "correct" to say "say-ing" and "incorrect" to say "say-in"… which means that if we want to sound like schoolteachers, we say "say-ing," but if we want to sound like down-home, unpretentious folks, we say "say-in."
So, there it is. If you want to show your knowledge and not sound ignorant, you should always pronounce things the way they're spelled, except for all the exceptions (like cupboard and knowledge), which you should not pronounce the way they're spelled lest you sound ignorant. So… which should it be, "suh-sinct" or "suck-sinct"?
It may come down to personal taste — perhaps "suh-sinct" sounds a little, uh, flaccid to you. Perhaps, with Charles Harrington Elster in his unabashedly opinionated Big Book of Beastly Mispronunciations, you declare "it will be a cold day in orthoepic hell" before you utter such a "slovenly" mispronunciation. Or on the other hand, perhaps, like one commenter at Throw Grammar from the Train, you've "always pronounced succinct as 'sussinct' and will say that aurally it exemplifies its definition better than its traditional pronunciation."
To be as succinct as possible: You might as well think of who you most want to impress (or least want to offend) and find out how they say it.
CORRECTION: An earlier version of this article implied one pronunciation of "flaccid" is more accepted by Merriam-Webster than the other. It has since been corrected. We regret the error.