Many nicknames are diminutives of proper names. William gets shortened to Will or Bill; Robert gets shortened to Rob or Bob; Johnathan gets shortened to John or Jack. Invariably, it's the first syllable of the name that is retained, and the end that gets lost.
And back when I was in high school, a couple of us opted to do things a bit differently, and to instead drop the first part of the name, and keep the end. My buddy Nathan, became Th'an rather than Nate. Greggory became Gory rather than Gregg. And I became Chard rather than Rich or Rick.
And in my case it simply stuck. My brother, sister and Ex-wife still call me Chard, as does my old buddy Ptrope (Our art forum mod, who I've known for over 30 years! Yikes!) So when it came time to pick a user name... Chard was pretty much the obvious choice. However in the early days of the World Wide Web, a lot of those early BBS demanded that a username be at least 7 or 8 characters long, and thus Chardman was born.