Tricked or outsmarted by someone clever; or in printing, stained with brown spots that appear on old paper.
From 'fox' as a verb meaning to deceive, dating to the 1600s, referring to the fox's reputation for cunning. The paper staining meaning is from the foxing spots' brownish color resembling a fox's fur.
Foxing on old books is a chemistry mystery—those brown stains come from metal compounds and mold, not from actual foxes, but medieval people thought the discoloration looked like the animal, and the name stuck for 500 years!
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