What is the world's longest word?

A difficult question. Some good candidates for English have already been noded up, but for most other languages you have to limit the question a bit more.

First of all, compound words are not fair game, since many languages (like German) can pile them up to near-infinite length. Alas, this disqualifies many family favorites like Hottentottenpotentatenattentat ("an attempt to assassinate a Pygmy regent"). Nearly all those really long place names are also compound words.

Second, chemical names are not really acceptable either, since they're essentially compound words built out of Greek, more or less equivalent to reading out "hydrogen and carbon and hydrogen and oxygen and carbon and hydrogen and..." in English. So we have to disqualify these too.

Next, we have the issue of agglutinative versus non-agglutinative languages. Instead of English-style helper words ("of", "by", "your", "through"...), agglutinative languages add prefixes and suffixes to word stems, resulting in hideously long words if you just conjugate them enough. One classic example is the Turkish Afyonkarahisarlılaştıramadıklarımızdanmısınız ("Aren't you one of those people whom we tried, unsuccessfully, to make resemble the citizens of Afyonkarahisar?") -- although this contain a proper noun and is thus a bit iffy. Some people disqualify these too, because you obviously won't find the conjugated forms in a dictionary, but most people think they qualify.

And thus, according to some editions of the Guinness Book of World Records, the world's longest word is the following Finnish monster:

epäjärjestelmällistymättömyydellänsäkäänköhän
But what does it mean?

Let's analyze, step by step:

epä "un-"
järjestelmä "system"; from järki, reason, via järjestää, to organize
llis adjective indicator
t causative, action from outside
yttä "-zation", becoming something
noun indicator
ttöm from -ttä, "-less", abessive case
yyde from -yys, "-ness", quality of being something
llä "using", adessive case
n "with", instructive case
3rd person possessive case
kään "also"
question indicator
hän "I wonder?"

Note: I'm not a grammarian and I've probably got some of those wrong, corrections are welcome. Finnish cases all conjugate happily into each other, mutating as they go along, so disentangling something as purposely perverse as this word isn't easy...

Yeah yeah, but what does it MEAN?

(I was hoping I wouldn't have to answer that.)

This is a sentence fragment, mind you, so I need to add a blank in the middle in order to transform it into English:

Wonder if he can also ... with his capability of not causing things to be unsystematic?
Or something along those lines. A very useful word indeed, yes?

Could you make something even longer?

But of course, and I believe some later attempts have made it past the 100-letter mark. The word above is undoubtedly the most famous of the bunch though, because unlike the longer candidates it still remains barely comprehensible.

However, Finnish also has a method of generating infinitely long non-compound words through a peculiar recursive verb construction. Here's an example, using the verb tehdä ("to do"):

N  Verb          Meaning
0  tehdä         to do
1  teettää       to have someone do
2  teetättää     to have someone have someone do
3  teetätättää   to have someone have someone have someone do
   ...
N  tee(tä)Nttää  to (have someone)N do
The N=2 form is occasionally used even in real life, but forms beyond that aren't spotted too often. Still, they are grammatically perfectly valid Finnish, and thus the N=∞ form is and shall remain the longest word in the world.

Much of this used to be noded under epäjärjestelmällistymättömyydellänsäkäänköhän, which screwed up softlinks with its length and was thus nuked.