Commenter Sparky reports, and the Oxford English Dictionary confirms: The term “taser” comes from “Tom Swift’s Electric Rifle.” “Yup, THAT Tom Swift,” writes Sparky; “He had an electric rifle in a 1911 story.”
Sparky (and Wikipedia) report that it comes from “Thomas A. Swift Electric Rifle” (Wikipedia: “The ‘A’ is gratuitous; the character’s middle name was never provided”), though the OED doesn’t say so.