A Lithuanian one, fain I`d say, but for our Language Functionaries declaring it is not. There`s many a word they deem "too much too Russian (or Slavonic, or something)", even though the words might well have been used and understood by Lithuanians long before the Functionaries were born.
Matter is, we have synonyms for "durnius", only none of the synonyms could fully replace the word. It has a sense of its own. The Russian "дурак" or "дурень" seem to have shades of their own, as well (a tad on that to be found here: http://www.translatorscafe.com/tcTerms/en-us/thQuestion.aspx?id=87645 ).
Anyway, I do not think words should be put under a ban for "patriotic" reasons. Especially words that have no full-fledged equivalents.
Ne viskas rusiška, ką rusai vartoja!
Šitaip dar ir "karvę", "varną" beigi "vartus" neteiktinais apšaukt galima...
Šiaip ar taip, "durnius" ir "kvailys" nėr vienas ir tas pat.