Thread:ShonenChicoBoy/@comment-25579547-20160928095712/@comment-18812574-20161022160832

According to Reddit:

"It actually has nothing to do with circles, roundness, or correctness. Maru is a suffix used for affectionate nicknames for children (usually boys), or the actual names of swords, ships, and pets. It comes from -maro (麿, an old first-person pronoun), and over time the pronunciation shifted to -maru. Since there was already a word pronounced maru, the two got grouped together using the same kanji. It basically means "something with a certain attribute". So if a young boy had bright white hair, he might be called Shiromaru (shiro being white). Hence: Zabimaru: "snake tail" (it looks like a snake tail) Hyourinmaru: "moon" (it controls weather like the moon) Hoozukimaru: "ground cherry" (the red tassel resembles a ground cherry) Gonryoumaru: "lightning" (strikes like lightning, might even have lightning abilities) Tengumaru: "Tengu" (this one's a mystery really -- it could refer to the fans Tengu sometimes wield, here being used to create/feed flames)"