the oldest profession

Prostitution. It's called the oldest profession, and it's probably true that it is!

wear (one's particular profession's) hat

To act as one would in one's particular profession while in a different setting. Bobby, I know you're off duty, but can you please wear your doctor's hat for five minutes and tell me what's wrong with my arm? I don't want to have to go to the hospital. My wife was still wearing her judge's hat when she tried to intervene with our neighbor's arguing kids.

the oldest profession

the practice of working as a prostitute. humorous

Politics or the law is sometimes humorously awarded the status of ‘second oldest profession’, with the sarcastic implication that their practitioners are as immoral and mercenary as society traditionally considered prostitutes to be.