Onund appears in the beginning of Grettir’s Saga, a 14thC Icelandic Saga, as a kind of genealogical backstory. A free version of the saga is available on the Saga Database here if you want to explore it more, based Morris and Magnusson’s translation, and for the purpose of accessibility this will be the translation used here.
The nickname ‘Treefoot’ stems from a physical injury obtained by Onund in a naval battle against Harald (later ‘Finehair) to oppose his conquest of Norway. With his lower leg cut off, Onund’s leg is replaced by a wooden peg-leg.
What is particularly interesting is the way the saga disputes the implications of the name, and the disability that Onund recieves. Onund himself clearly sees the name and impairment as a negative reflection on his ability and masculinity, declaring:
‘”What joy since that day can I get
When shield-fire’s thunder last I met;
Ah, too soon clutch the claws of ill;
For that axe-edge shall grieve me still.
In eyes of fighting man and thane,
My strength and manhood are but vain,
This is the thing that makes me grow
A joyless man; is it enow?“
(https://sagadb.org/grettis_saga.en)
A similar attitude is expressed by a set of taunting Viking:
“Treefoot, Treefoot, foot of tree, Trolls take thee and thy company.”
“Yea, a sight it is seldom seen of us, that such men should go into battle as have no might over themselves.”
(https://sagadb.org/grettis_saga.en)
Interpreted in this manner, the nickname is clearly a tool to mock Onund’s poor skill or luck in combat, and to degrade his impairment. However, the Saga replies with a remarkable respectful and understanding exploration of disability, and makes clear that Onund is not to be understood as a lesser person because of his physical impairment: after his self-doubt, Thrand replies to Onund that ‘whereso he was, he would still be deemed a brave man‘ (https://sagadb.org/grettis_saga.en). This ambiguity of translation is a common feature in nickname studies, especially within the early medieval period, but might represent an actual reality – might the nickname have meant different things with different implications to different groups, friends or enemies?