The interesting thing about this name is that it's unclear if the first part, "mod", is actually part of the name. Without it, the name would just be Thryth (which could also be submitted as a hypothetical name). Either way, the character and description of Modthryth in Beowulf has been interpreted as an attack or insult against Cynethryth, the powerful queen consort of Offa of Mercia. https://en.wikipedia.org/wiki/Modthryth https://en.wikipedia.org/wiki/Cynethryth
https://en.wikipedia.org/wiki/Modthryth
https://en.wikipedia.org/wiki/Cynethryth