A hard on is sex slang for a male erection. When a man has a hard on, he is sexually aroused. This causes his penis to become hard and erect.
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A hard on is a term used in popular culture to describe the hardening of the penis when a man is sexually aroused. When stimulated or aroused, the male penis becomes stiff and erect. This is why a hard on is used to describe the erect penis.
