etymology - Does "syllabus" derive from Greek or Latin?

etymonline to the rescue:

syllabus (n.)
1650s, "table of contents of a series of lectures, etc.," from Late Latin syllabus "list," ultimately a misreading of Greek sittybos "parchment label, table of contents," of unknown origin. The misprint appeared in a 15c. edition of Cicero's "Ad Atticum" (see OED). Had it been a real word, the proper plural would be syllabi.

So, it seems that it is based on a misspelling of a Greek word - but that would hardly give one reason to form the plural in Greek starting form the Latin(ized) form.

According to Merriam-Webster the plural is syllabi or syllabuses, but with etymonline's no real word-verdict, I'd go for the latter.