Mephistopheles

Etymology

The two-time occurrence of -ph- which is the Roman transliteration for Ancient Greek φ and of the termination -es which transliterates Ancient Greek -ης as in Aristoteles (Ἀριστοτέλης) along with the resemblance -phel-/-phil- bears to φιλεῖν (phileîn, “to love”) has led many to believe that this must originally be a Greek compound word. Based on this assumption, the first two letters have been identified with μή (mḗ, “not”), while -phisto-/-phosto- has been interpreted as a corruption of φωτο- (phōto-), the compositional form of φῶς (phôs, “light”). The name would thus mean "not loving light" which seems fitting for a devil. However, there are two major problems with this theory. Firstly, phōto- is a common and clearly recognizable morpheme in Greek, making it an unlikely candidate for corruption. Secondly, μή (mḗ) is not typically used in Ancient Greek name formation.
A more credible explanation has been proposed by Julius Goebel. He focuses on the alternative form Mephistophiel, which appears in the Praxis Cabulae nigrae Doctoris Johannis Fausti magi celeberrimi (1612). He interprets mephist- as a disguised form of megist-, transliterating μέγιστος (mégistos, “greatest”), an epithet of Hermes in the title of Ἑρμῆς Τρισμέγιστος (Hermês Trismégistos, “thrice-greatest Hermes”) who was the guardian-god of the magicians, astrologers and alchemists. The second part, -ophiel, is interpreted as the proper name Ophiel, composed of ὄφις (óphis, “serpent”) and -ήλ (-ḗl), which is the Hebrew אֵל (“god, deity”). The derivation of demon names in -el from non-Hebrew bases was common in magic-books of the 16th and 17th century; other examples include Kyniel from Greek κύων (kúōn, “dog”) and Ariel from ariēs (“ram”).
Ophiel thus means "serpent-god" and refers to Hermes, who wields the κηρύκειον (kērúkeion), a staff entwined with two serpents. The appearance of Ophiel in the Wagnerbook and the Arbatel, where it is listed as another name for Mercury, supports this interpretation. Goebel therefore identifies Mephistopheles with Hermes Trismegistos and the planetary figure of Mercury.
Wilhelm Roscher proposed a similar explanation for the name in his Ephialtes-monography about demons, where he states the name to be traceable back to *Μεγιστωφέλης (*Megistōphélēs), from μέγιστος (mégistos), as stated above, and ὠφελέω (ōpheléō, “to help”).https://archive.org/details/ephialteseinepa00roscgoog/page/n102/mode/1up