Do you now the longest Hungarian word?

No

I don't know the word but I noticed that people have downrated you for asking this question and I don't understand why. It doesn't seem fair to me that a simple question like this should get a -4...so I'll give you my +2.

Since Hungarian is an agglutinating language (it adds suffixes morphologically to the end - and sometimes beginning - of the word), there are countless ways of constructing absurdly long words. My two favorites, and these or their slightly changed counterparts are the most commonly cited, are as follows: elkelkáposztástalaníthatatlanságoskodásaitokért and megnemszentségteleníthetetlenségeskedéseitekért (took about two mintues to write these down...). But you would hardly find anyone using these words in common speech, only as tongue-twisters. I would guess that the more widely used long words are typically terminus technicus, but most of these are divided by a dash (because of a grammatical rule, which is not active in the case of the previous two examples). But we do have common long words, these come to my mind first: igazságtalanság (injustice - this and any other word can be elevated to the same longish level by adding suffixes such as personal, plural, tense, etc.), libamájpástétom (goose-liver paste), kényszerleszállás (forced landing), gyógyszerészhallgató (pharmaceutical studies student), rabszolgakereskedelem (slave-trade); these are all dictionary-words, while the two earlier samples are not.

Mine is megkarimásithatatlanithatnátók

elkaposztasithatatlanitottatok (it is my longest Hungarian word with some accents)

alegeslegmegszentségteleníthetetlenségeskedéseitekért