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.

The full OED entry for the word mentioned by Etymonline is much too long to fully cite here, but includes the following material pertinent to the current discussion:

syllabus (n.)

Etymology

< modern Latin syllabus, usually referred to an alleged Greek σύλλαβος. Syllabus appears to be founded on a corrupt reading syllabos in some early printed editions — the Medicean MS. has sillabos — of Cicero Epp. ad Atticum ɪ. iv, where the reading indicated as correct by comparison with the MS. readings in ɪ. v. and viii. is sittybas or Greek σιττύβας, accusative plural of sittyba, σιττύβα parchment label or title-slip on a book. (Compare Tyrrell and Purser Correspondence of Cicero nos. 107, 108, 112, Comm. and Adnot. Crit.) Syllabos was græcized by later editors as συλλάβους, from which a spurious σύλλαβος was deduced and treated as a derivative of συλλαμβάνειν to put together, collect (compare ꜱʏʟʟᴀʙʟᴇ n.).

In the passage from S. Augustine’s Confessions xɪɪɪ. xv. (‘ibi legunt [sc. angeli] sine syllabis temporum quid velit aeterna voluntas tua’) commonly adduced as further evidence of Latin syllabus, the word is clearly syllaba syllable.

The earliest provided citation dates from a 1656 work by antiquary and lexicographer Thomas Blount in his Glossographia; or, A dictionary interpreting all such hard words, whether Hebrew, Greek or Latin... as are now used in our refined English tongue (1ˢᵗ edition, 1656, London), where he writes:

• Syllabus, a Table or Index in a Book, to shew places or matter by Letters or Figures.

This was quickly followed in 1667 by Jeremy Taylor, Church of Ireland Bishop of Down and Connor, in the 4ᵗʰ edition of his work The great exemplar of sanctity and holy life (1ˢᵗ edition 1649, 4ᵗʰ edition 1667), where he writes at ɪ. vi. §22. 160:

• The Apostle expresses it still by Synonyma’s, Tasting of the heavenly gift, and made partakers of the holy Ghost..; all which also are a syllabus or collection of the several effects of the graces bestowed in Baptism.

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 from the Latin(ized) form.

According to both Merriam-Webster and the OED alike, the plural can be either syllabi /ˈsɪləbaɪ/ or syllabuses /ˈsɪləbəsɪz/, but with Etymonline’s “no real word” verdict, I’d go for the latter.

This word entered the Modern English vocabulary through late Middle English: from an Anglo-Norman French alteration of the Old French word sillabe, via Latin from Greek sullabē ('συλλαβή') a compound word coming from the preposition 'syn' - meaning plus or together ('συν') + a form of the verb 'lambanein' ('λαμβάνειν') to receive or take in Greek. The Greek preposition 'syn' is a compound of many English words (synchronous, synthesis, synonym, synergy, syntax, syncope, synagogue) to denote actions that happen either concurrently or one is a consequence of the other. All of the above words, alongside many others, are of Greek origin and have travelled a long way through time and space from Southeastern Europe to the Northwest. Another interesting use of the preposition 'syn' is in the word asynchronous meaning not existing or occurring at the same time, which also includes the prefix 'a-' to express absence or negation.