CHAT : Origin of the name "Northumbria"

I ended up in a discussion of this on a gaming forum, arguing it's the kind of
name one'd expect to be given by othersiders rather than a self-assumed one;
for the Northumbrians themselves, living north of the Humber probably did not
seem like the most noteworthy thing about them.

Is it known where the name originates?

                                                     Andreas

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