In Italian it is generally considered "precipitevolissimevolmente" (26 letters). This word was coined in 1677 by a poet, Francesco Moneti, which wanted a word long as an hendecasyllable. It is the superlative of the adverb "precipitevolmente", which is in turn derived from the adjective "precipitevole". In the end it means something like "very hurriedly", "headlong", "precipitously". However the poet cheated, since the correct superlative would have been "precipitevolissimamente". By the way, it was used again and again until it ended up in Italian dictionaries (it is used only in joking contexts).

Donaudampfschifffahrtsgesellschaftsraddampferkapitänskajütentürsicherheitsschlüssel

Not that bad either...