I was surprised to find out that the Arum Lily is neither from the Arum nor Lily genera. According to Gardening Made Easy this particular plant is named Arum Lily because the flower's appearance represents purity (and elegance).
An amusing fact that I discovered was that the Arum Lily is called a Varkoor (translated as Pig's Ear) in Afrikaans. Could the English and Afrikaans names have more contrasting meaning?! Purity, elegance, and a pig's ear. :)