I'm glad I know sign language, it's pretty handy.
Why this joke works
Why Sign Language Is Handy
Signed languages communicate through coordinated hand shapes, movements, facial expressions, and body position; they are complete natural languages, not word-for-word pantomime. “Handy” means useful or convenient, but it also points literally to the hands used in many signs. The joke’s compliment therefore works in two ways. Its best reading respects sign language as a real language while using the ordinary adjective’s anatomical association.
A closer look
The Compliment Points to the Language’s Visible Articulators
“Handy” normally means useful or convenient, making the sentence sound like simple appreciation for a learned skill. American Sign Language prominently uses hand movements, so the adjective also describes the body part involved. The joke’s first-person wording avoids treating signing as a novelty; it praises genuine knowledge while letting one everyday word carry a literal physical association. The comma gives the payoff a relaxed afterthought quality.
Fact behind the joke
American Sign Language Is a Complete Natural Language
The National Institute on Deafness and Other Communication Disorders explains that ASL has its own grammar and the fundamental properties of spoken languages. It is expressed through coordinated movements of the hands and face, includes fingerspelling, and varies through regional accents and dialects. Sign languages are not universal: British Sign Language and ASL, for example, are distinct languages rather than interchangeable hand versions of English.