1.
claim that one has (a quality or feeling), especially when this is not the case.
"he had professed his love for her only to walk away"
synonyms: declare, announce, proclaim, assert, state, affirm, avow, maintain,
protest, aver, vow;
2.
affirm one's faith in or allegiance to (a religion or set of beliefs).
"a people professing Christianity"
synonyms: state/affirm one's faith in, affirm one's allegiance to, make a public declaration of, declare publicly, avow, confess, acknowledge publicly
"in 325 the Emperor himself professed Christianity"
From Latin "profiteri", to the form "profess-" meaning "declared publicly", and to "professor", then to Late Middle English as "professor".
So a professor's practice is probably closer to definition 2: "make a public declaration of" whatever one's skill or knowledge of a particular art might be.
profess
/prəˈfɛs/
verb
1. claim that one has (a quality or feeling), especially when this is not the case.
"he had professed his love for her only to walk away" synonyms: declare, announce, proclaim, assert, state, affirm, avow, maintain, protest, aver, vow;
2. affirm one's faith in or allegiance to (a religion or set of beliefs).
"a people professing Christianity" synonyms: state/affirm one's faith in, affirm one's allegiance to, make a public declaration of, declare publicly, avow, confess, acknowledge publicly "in 325 the Emperor himself professed Christianity"