I'm not sure I recall the exact moment when I renamed her. It was probably one of those struggling adolescent moments when the pre-teenage boy was at a crossroads, having always been exceptionally close to his mother but reaching a phase where his friends found this closeness uncool.
Or perhaps I was emulating her- she had a lifelong habit of nicknaming her friends and relatives. I was nicknamed Skip at age five, due to a note on my Kindergarten report card. Dee the Bricklayer, Aunt Hussy, General Letty, all of these names were part of my everyday life. The advent of "handles" during the CB radio fad had nothing on her.
So I started calling her Chuck. And she answered to it. When written communications demanded a bit more dignity, birthday cards were addressed to Lady Charles of Southhampton. Her cards to me were signed simply Chuck. The exact origin of Chuck is also unclear, her theory was that it was a reference from M*A*S*H, but upon reflection I think I was referencing Peppermint Patty's nickname for Charlie Brown.
Either explanation could descend into way too much thought. Maybe it was just a away for a teenage boy to remain close to his Mom while simultaneously needing to grow up and pull away.
But as tends to happen with adolescents, it was just a phase. Once the Preppy Handbook was published, I renamed her Muffy.