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Hemingway's prose was bad writing.
Literal
Hemingway [genitive-の] prose [topic-は] bad-prose was.
A provocatively opinionated literary judgment. 悪文 (あくぶん) is a compound: 悪 (bad) + 文 (writing, prose) = 'bad writing,' 'poor prose style.' The opposite is 名文 ('excellent prose'). This is literary-critic vocabulary — you wouldn't use it casually. The past tense でした is interesting here because Hemingway's prose still exists; the past tense reflects a scholarly/historical framing ('in the context of when it was written/reviewed'), not that the prose has disappeared. This is a sentence you'd find in a literary-criticism essay or opinion piece.