Of uncertain origin.
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 ” ) .