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).
