A beautiful boy. The allusion is to Adonis, who was beloved by Venus, and was killed by a boar while hunting.
Rose-cheeked Adonis hied him to the chase; Hunting he loved; but love he laughed to scorn. Sick-thoughted Venus makes amain unto him, And, like a bold-faced suitor, gins to woo him.
—Shakespeare: Venus and Adonis.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894