While historians have several explanations for Ivar’s perplexing title (e.g, that it’s ironic, or that it’s a comment to his agility in battle, or a reference to his impotence, or a mistranslation of the Latin word for “detested,” as outlined in University Press’ “Ivar the Boneless: Viking Warrior”) many have interpreted it to mean he had brittle bone disease, otherwise known as osteogenesis imperfecta.
This literal reading of the name stems largely from a passage in The Saga of Ragnar Lothbrok, commonly translated as follows: “… the boy was … given the name Ivar. But the boy was boneless, and it felt as though there was only gristle where the bones would be” (Crawford, J. 2017 “The Saga of the Volsungs with The Saga of Ragnar Lothbrok,” page 97). It’s this most literal understanding that Hirst chose to use in his depiction, and that, by all appearances, prompted him to write in Ivar’s alternating eye-color. In the series, Ivar’s eyes are an indicator of his physical vulnerability. The bluer they appear, the more likely he is to break a bone. There are a handful of references to this phenomenon, including a discussion between Ivar and Ubbe in Season 5, Episode 8, wherein Ivar says, “You remember I had to ask you ever single morning how blue the whites of my eyes were, because if they were very blue I was in great danger of breaking a bone.”