A cuckold is a husband whose wife has had sex with someone else. By doing this, she cuckolds him.
This is an old-fashioned word you can find in many Shakespeare plays, though cuckolding is certainly older than Shakespeare and will always exist as long as there are marriages. A man can't cuckold a woman: only a wife can cuckold her husband. By sleeping with another man, she makes her husband a cuckold. Believe it or not, some husbands actually like being a cuckold and encourage their wives to sleep around: these people have a cuckold fetish.
