The longest word in any given language depends on the word formation rules of each specific language, and on the types of words allowed for consideration.
Megszentségteleníthetetlenségeskedéseitekért, with 44 letters is officially the longest word in the Hungarian language and means something like “for your [plural] continued behaviour as if you could not be desecrated”. It is already morphed, since Hungarian is an agglutinative language.
According to wikipedia.com, partition to root and suffixes with explanations:
meg- verb prefix; in this case, it means “completed”
szent holy (the word root)
-ség like English “-ness”, as in “holiness”
-t(e)len variant of “-tlen”, noun suffix expressing the lack of something; like English “-less”, as in “useless”
-ít constitutes a transitive verb from an adjective
-het expresses possibility; somewhat similar to the English modal verbs “may” or “can”
-(e)tlen another variant of “-tlen”
-ség (see above)
-es constitutes an adjective from a noun; like English “-y” as in “witty”
-ked attached to an adjective (e.g. “strong”), produces the verb “to pretend to be (strong)”
-és constitutes a noun from a verb; there are various ways this is done in English, e.g. “-ance” in “acceptance”
-eitek plural possessive suffix, second-person plural (e.g. “apple” → “your apples”, where “your” refers to multiple people)
-ért approximately translates to “because of”, or in this case simply “for”
Words of such length are not used in practice, but when spoken they are easily understood by natives. They are not compound words—they are formed by adding a series of one and two-syllable suffixes (and a few prefixes) to a simple root (“szent”, saint or holy). There is virtually no limit for the length of words, but when too many suffixes are added, the meaning of the word becomes less clear, and the word becomes hard to understand, and will work like a riddle even for native speakers.