William Armstrong got his unusual nickname from his older brother who thought he looked like a doodlebug when he tried to crawl as a baby. Doodle was born with birth defects that included a large head and a small body. When his parents name him William Armstrong, Brother remarks that it is “a name [that] sounds good only on a tombstone,” foreshadowing the eventual death of Doodle. When Doodle starts to crawl, Brother describes him and says,
When he crawled, he crawled backwards, as if he were in reverse and couldn’t change gears. If you called him, he’d turn around as if he were going in the other direction, then he’d back right up to you to be picked up. Crawling backward made him look like a doodlebug.
A doodlebug is an insect indigenous to the region in which the story takes place. The doodlebug is found in Texas around the Gulf coast. Brother would have seen many doodlebugs in his time; and therefore, the description appears to be appropriate.
A footnote in the copy of my story mentions that a doodlebug is also a train that goes backwards and forwards. Again, this is a fitting description of what Doodle looks like when he crawls.
So, William Armstrong earns the nickname, Doodle, simply because of his physical limitations.