A hand gesture used by metal heads to symbolize their love of the music. First used by Ronnie James Dio during his years in Black Sabbath, during the Heaven and Hell tour.
Dio would use the horns to emphasize a particularly dark moment. The moment had to deserve the horns. Dio himself did not invent the sign, the horns were originally an italian gesture that was used either to repel or to give the evil eye, and went by the name Mano Cornuto. Dio gives the credit to his grandmother for impressing the horns upon him, for she was the one who used it the most around him.

when you're hungover as shit after a spending a night drunk as a skunk

An English expression for an erection.

It's a phrase you use when you're referring to the horns of a thing with horns.

A hand gesture in which the pinky and index fingers are raised. This is different from the American Sign Language gesture for "I Love You" in that the thumb is tucked in; and different from the sign of the evil eye in that the fingers are pointed upwards. Usually used in celebration of something or other.

Euphemism for an erection/the process of getting an erection

The horn just means you have the fucking horn! Having the horn refers to a want in sexual desire. The horn-bearer wants it. Sluts have the horn and will take such horn. Most guys have the horn as general rule. If out at bars its always a horn-off, (often referred to as flirting), yet one must be cautious in horn judgement as its never true love. The horn for males can extend to various and great lengths.