Infested with fleas, or shabby and decrepit (often used to describe animals, buildings, or general conditions that are in poor shape).
Combining 'flea' and 'bitten' (past participle of bite), used since the 1600s. Originally literal, it evolved to mean any shabby or neglected thing.
A 'fleabitten' horse became the ultimate Shakespearean insult—to compare something to a mangy, flea-infested animal was the worst visual diss available before modern slang!
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