prof (n.)

late 14c., professour, "one who teaches a branch of knowledge," especially in a university, from Old French professeur (14c.) and directly from Latin professor "person who professes to be an expert in some art or science; teacher of highest rank," agent noun from profiteri "lay claim to, declare openly" (see profess). As a title prefixed to a name, it dates from 1706. Short form prof is recorded from 1838.

Professor. One professing religion. This canting use of the word comes down from the Elizabethan period, but is obsolete in England. [Thornton, "American Glossary," 1912]

This sense is traced in OED to 1530s, but is perhaps a revival by the English Puritans of the use of the word from c. 1400 in the sense of "one who openly professes religious faith."