This word is a noun meaning simply 'disuse'. It is sometimes used in academic texts. The simpler word is better English in the opinion of AWE. When desuetude is used, it is usually in the construction 'has fallen into desuetude,' for 'has fallen into disuse' or 'has become unused'.
Desuetude has two different pronunciations. The more traditional in the UK is a three-syllable word with the main stress on the first syllable and a short '-i-' sound in the second, 'DES-wi-tyoud' IPA: /ˈdɛswɪtjuːd/. In the US, and for some speakers in Britain, it has four syllables with the main stress on the second, which has an '_oo_' sound, 'der-SOO-et-ood' IPA: /dɪsˈuɪtuːd/.
There is a rare adjective desuitudinous.