Brown or tan spots and stains that appear on old paper, books, or documents, usually caused by moisture and age.
From 'foxing' (1840s origin uncertain, possibly from the reddish-brown color resembling a fox, or from chemical reactions). The term emerged in the 19th century to describe discoloration patterns found in aging documents.
Book lovers call these stains 'foxing,' but nobody knows for sure why—the most popular theory is that the rust-colored spots reminded people of a fox's fur, but it could also be a chemistry thing we've lost the memory of. Collectors actually pay more for books with interesting foxing patterns because it tells you the book's whole life story.
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