Chirologia, or the Naturall Language of the Hand, 1644

John Bulwer was a medical practitioner and, essentially, an early linguistic theorist who explored the idea of the physical human body as a medium of communication. With Chirologia, Bulwer created one of the first English books on deafness and the education of deaf-mutes. Chirologia comprehensively catalogs the meanings of hand gestures and emphasizes the value of manual gestures for speech, oration, and acting. His catalog of gestures is not based on a set "sign language," but rather his own observations and other classical texts. However, Bulwer did advocate for special schools for the deaf, although he was really more interested in devising ways of teaching the deaf to speak than in designing, describing, or using any sign language they might have of their own. [source, source]

Bulwer argued that gestural language was universal and primary, while spoken language is just one more tool in the complex scheme of communication. His idea related to the contemporary interest in the notion of universal languages, as well as supporting what would later be known as the gestural theory, which proposes all language evolved from gesture.

You can find more information on the book at the Folger Shakespeare Library and an you can check out an excerpt included in the 2001 book Imagining Language: An Anthology, which is a collection of writings that "demonstrate the continuum of creative conjecture on language from antiquity to the present." So apparently, what I first thought of as funny Renaissance gang signs turned out to be a pretty important step for the instruction of the deaf and a prescient take on linguistic evolution. This makes me feel slightly bad for making a handjob joke.