This is a reference to a "deaf joke." When I was young, the kids in school would tell Helen Keller jokes. Helen Keller, for those who don't know, was deaf, dumb and blind. The jokes were along the lines of "What did Helen Keller do when she fell in the well? -"She screamed her hands off." (instead of screaming her head off, as the idiom normally goes, since deaf people make signs with their hands instead of forming words with their voices.) Here the speaker probably is deaf and has deaf friends, and I imagine they went to a comedy show and his friends were "laughing their hands off" (instead of laughing their heads/asses off).