sophism n.

"one who makes use of fallacious arguments," late 15c., from Late Latin sophista, an alternative form of sophistes; the earlier form in English was sophister, sophistre (late 14c.). Latin sophistes is from Greek sophistēs "a master of one's craft; a wise or prudent man, one clever in matters of daily life," from sophizesthai "to become wise or learned," from sophos "skilled in a handicraft, cunning in one's craft; clever in matters of everyday life, shrewd; skilled in the sciences, learned; clever; too clever," a word of unknown origin.

Greek sophistēs came to mean "one who gives intellectual instruction for pay," and at Athens, contrasted with "philosopher," it was a term of contempt.

Sophists taught before the development of logic and grammar, when skill in reasoning and in disputation could not be accurately distinguished, and thus they came to attach great value to quibbles, which soon brought them into contempt. [Century Dictionary]

The "learned man" sense in English is attested from 1610s but is rare.

"subtle but fallacious reasoning," mid-14c., sophistrie, from Old French sophistrie (Modern French sophisterie) and directly from Medieval Latin sophistria, from Latin sophista, sophistes (see sophist).

"Sophistry applies to reasoning as sophism to a single argument" [Century Dictionary]. The meaning "sophistry; the philosophy, methods, and teachings of the Greek sophists" is attested by 1837; sophisticism in this sense is by 1889.

1680s, "student in the second year of university study," literally "arguer," altered from sophumer (1650s), from sophume, an archaic variant form of sophism, ultimately from Greek sophistēs "a master of one's craft; a wise or prudent man, one clever in matters of daily life."

The modern form probably is by folk etymology derivation from Greek sophos "wise" + mōros "foolish, dull" (see moron), "as if in allusion to the exaggerated opinion which students at this age are apt to have of their wisdom" [Century Dictionary]. The 17c. -er is perhaps based on philosopher, sorcerer, sophister, etc.

The original reference of the "arguer" name might be to the dialectic exercises that formed a large part of education in the middle years. At Oxford and Cambridge, a sophister (from sophist with spurious -er as in philosopher) was a second- or third-year student (what American colleges since mid-18c. would call a junior might be a senior sophister).