Displaying loose morals or sexual behavior, or more neutrally, slovenly or untidy in appearance.
From Middle English 'slut' (a slovenly or immoral person), possibly from Old Norse 'slottr' (a sloven). The modern sexual meaning developed in the 15th century.
The word 'slut' originally just meant 'messy' or 'slovenly'—the sexual meaning came later and is one of the most gendered insults in English, since the same behavior is often praised in men but condemned in women.
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