<SFX>Old guy, 95 years, no teeth
Many, many years ago, before the internet was popular, when BBS' were all the rage, I signed on to High Society BBS in Fort Lauderdale.
I just happened to see the BBC version of the Hitchhiker's Guide on PBS just a few weeks earlier. Zay was cool.
Someone had already picked "zaphod", then abandoned the account. Since the owner wanted to boast of having thousands of subscribers, he'd never, ever delete any screen name, ever, period.
Therefore, "Zayphod" came into play (pronounced the same anyway). This was back around 1983 or so. Kept the screen name ever since (on my AOL account, too).
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