The ONP page for Galdrahríð has definitions from Cleasby & Zoega, and lists all the words containing the headword Galdra, including this one.
galdra-kinn, f. a ‘spell-cheek,’ a nickname, Eb.
Other entries have a citation that can be clicked, but this one, Eb. can not be. What is it referencing?
I found the answer moments after writing the question, but I thought I'd add a self-answer in case this trip anybody else up in the future (like me, when I forget how this works).
You can search the 'Works' tab for abbreviated references, like Eb.
Here Eb. refers to the Eyrbyggja saga.