A nickname is something given to you by other people. With the dawning of the internet age a person can have a certain amount of control over what they are called, but someone can really do with an internet handle is narrow down the choices other people have. For instance, when I first hit the BBS scene, pre-internet, I went by Wintermute, an ostentatious character name I had absolutely no reason to adopt, other than I thought it was cool.
Within a few months of getting involved in the local BBS scene, I was known as Wintermuffle, largely due to an incident with my first wife in a closet during a birthday party. Years later everyone just knows me as Fahey.
Or Big Dog.
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