A student at Cambridge is a Freshman for the first term, a Junior Soph for the second year, and a Senior Soph for the third year. The word Soph is a contraction of “sophister,” which is the Greek and Latin sophistes (a sophist). At one time these students had to maintain a given question in the schools by opposing the orthodox view of it. These opponencies are now limited to Law and Divinity degrees.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894