Well, I've gone through 4 username "phases".
My first username was "Koolkatkid10", Which I was using from 2008 to 2012.
Then I started a YouTube channel, and created the username "HeroSlayer X", which is a shorter form of "Herobrine Slayer X" which I used from 2012 to 2015. Then I chose to make a name that wasn't very "childish", so I made the username I use on most of my other accounts: "Sub-Optimal", because my brother "familyalle1" was hard at work trying to destroy my self-confidence. (We have a pretty unhealthy brother-sister relationship.) Which I used from 2016 - present day. Then comes to my current minecraft IGN: "MildlyShrill". There's not a whole lot of special meaning with it, I just needed a more feminine username. XD