I know there’s a few theories about Ivarr’s nickname. The obvious and one depicted in the show Vikings was that he had a bone disease that rendered his legs useless. However, I’ve also read that “boneless” was referring to his impotence. Some also think it was a simple mistranslation over time and “Boneless” isn’t the correct nickname at all.
So, is there a general agreement among historians on the most likely meaning behind the name?
There is no consensus, unfortunately. Ívarr really isn't called "boneless" in the English sources, and nothing in them suggests that he isn't able to walk.
So, the two main readings (disability and impotence) are derived from the Norse legendary tradition from several centuries later. The short saga (þáttr af Ragnarssonum) says that he wasn't able to sleep with any women, but he was exceptionally cruel, while the longer, later Ragnars saga says that he had no bones but also could do magic in order to sometimes fly through the air and pulling out the Rock's People's Elbow to crush the Swedish demon-cow Sibilja. I'm not joking.
Unfortunately, we obviously can't take the 13th century legendary tradition as having much of a relationship to the 9th century historical figure, especially without any corroborating evidence from the contemporary sources. So, it's left entirely speculative as to how he got the name "boneless".