He got the nickname just a few days after he was born. His parents brought him home, and his older brother Timmy got a look at him, and said, "Oh, he is such a beauty." Except that my Uncle Timmy was very small, and couldn't say "beauty" properly. It came out "Oh, he is such a BOOTIE." The name stuck.
My grandfather was "Bootie" all through his childhood, but as he grew so did the name, and by the time I came along, he had been Boots for many years.
Even then, I just called him Grandpa.