Although one definition is a vulgar way to describe sexual desire, horny also refers to something either horned or made of an animal’s horn or a similar hard material, as presented by John Singer Sargent’s “horny hand of toil.”
The word horny as describing the bony projection on an animal's head appeared in English before the 10th Century. A second meaning — that of being sexually aroused — appeared later. The two meanings connect in that the latter definition refers to male physical arousal. This was crudely referred to as "having the horn” in the late 18th Century before becoming the word horny in the mid-19th Century. First used exclusively for men, the word now applies to women's sexual feelings as well.