As far as I can tell, the double-x did not infiltrate the English language until the internet went mainstream (Exxon might be the exception—a company name deliberately chosen to be inscrutable and inoffensive). The earliest example I can think of is “anti-vaxx”, as in “anti-vaccine” (see also “antivaxxer”). Then we got “doxxing” (to publish someone’s personal information without their consent). And now we have “looksmaxxing”, which has itself been Watergated (i.e., “maxxing” has become a generic, often humorous suffix to attach to other words, such as “benchmaxxing”).
But where did the OG “anti-vaxx” come from? This 2018 post from Grammarphobia traces the term from its precursors “anti-vac”/“anti-vacc” in the late 19th century all the way to its popularization in the aughts. The Oxford English Dictionary dates the earliest known use of the double-x spelling of “vaxxed” to 2004.
Update 2026-08-19: Thank you to Tristan Miller, who wrote to point out a very conspicuous category of double-x usage I somehow overlooked in this post: leetspeak. Long before we were talking about “vaxx” and “anti-vaxx”, hackers were using x-heavy spellings like “haxxor”. As a kid in the mid-2000s, I was no doubt using terms like this to refer to my opponents in online games like Battlefield 2.
“Haxxor” has an entry in the Urban Dictionary dating to 2003, but the origins of leetspeak stretch back all the way to the dial-up BBSs of the 1980s. A diligent search of whatever archives remain of these forums and their descendants would probably turn up much earlier examples of the double-x. Of course, this is not proof positive that leetspeak inspired the double-x in “vaxx”, but it is valuable context nonetheless.