My name's Forrest,
Forrest Gump. My main obsession back when I first got on the internet was the video game Marathon (from Bungie, the makers of the Halo franchise, back before they sold their soul to the devil from Redmond).
The main enemies in Marathon are an alien race called the Pfhor, and many puns were made on the word "for" and words beginning with the syllable "for". Naturally, perpetrating such a pun on my name was an obvious choice of username.
Because "Pfh" is such an uncommon combination of letters and "Forrest" is a relatively rare name to begin with, I'm almost always guaranteed to have "Pfhorrest" available anywhere I need a username.
Someone else got the Twitter account "Pfhorrest" before me, but I don't care to use Twitter; that person also uses it in a few other places I have no interest in, such as airsoft and fountain pen forums. So it's generally not a problem. Turns out, oddly enough, he is not named Forrest himself, but he is a fan of Marathon, and has heard of me and my work in that fandom (one of the most popular mods was my creation), but under my real name, not my username.