The very first time I needed to create an on-line account was way back in the early 1990s (or possibly the late '80s). This would have been for the CIX conferencing platform. I tried 'Charlie' and 'Charlotte', but they were both already taken. I tried adding my other initials, but that didn't help. I was reluctant to add a number to my name (the usual solution at the time) as I'd never remember it, so I looked around the room for ideas.
My eyes settled on my copy of Charlotte Sometimes by Penelope Farmer. This had been my favourite book as a child and I still love it now. The full title was too long, so I needed to shorten it. Charlotte immediately became Charlie, but how to shorten Sometimes? I thought back to my degree in mathematics and decided on 'sum' for 'some', and 'x' for 'times', giving me 'charliesumx'.
Over the next few years, whenever I needed a name for a new account, I'd try 'Charlie' and 'Charlotte' first (which were always taken) before trying 'charliesumx'. But gradually, I found myself trying 'charliesumx' first... and I've never found it's taken (except when I'd previously created an account, then forgotten about it).
I do sometimes wish that charliesumx was a bit easier to read though (I've lost count of the number of people who've mis-read it as 'Charlie's Mum'), but after 30 years it's as much a part of me as Charlie and Charlotte.