A less than complimentary term for engaging in sex, more specifically, penetrative sex.
Considered a rude form of describing one's sexual act, in that it makes a loose comparison to how animals have sex.
It can also refer to a person masturbating, though more colorful words are usually chosen for that purpose.
The rubbing of one's genitals against another person, can also refer to having a hump, though there is no penetration or is being naked a requirement. Can be considered a form of foreplay.
Noteworthy: Derived from the way another will 'mount' another person, in that they are above and hunched over, creating a hump appearance, as they engage in copulation, sex.
The term dates back to the 18th Century, and is making a comeback in use, to mean having sex (copulation).