miswriting a word; using the wrong kanji
A yojijukugo referring to kanji copying errors, especially confusing similar-looking characters. The four kanji in the phrase themselves are easily mistaken for one another (魯 vs 魚, 亥 vs 豕). Used in discussions of classical texts and calligraphy.
See also: 魯魚の誤り (ろぎょのあやまり)
Many copying errors like 魯魚亥豕 can be found in old manuscripts.
A more common phrase for the same concept, literally 'the mistake of 魯 and 魚'. 魯魚亥豕 is the four-character compound version, more formal and literary.
From the classical Chinese story in 抱朴子 (Baopuzi) where the characters 魯 (foolish) and 魚 (fish), and 亥 (boar) and 豕 (pig) were confused due to their visual similarity. The phrase entered Japanese as a yojijukugo for copying errors.