Behaving in a way that people in your line of work would not accept. It is a judgement against a standard, and the standard belongs to the job, not to the person speaking — so the same act can be unprofessional in one job and fine in another. A doctor discussing a patient's illness at dinner is being unprofessional; a chef describing tonight's fish at dinner is doing their job. Two kinds of thing get called unprofessional. The heavy kind is a real breach of a job's rules — a lawyer using a client's money, a nurse turning up drunk — and this can end a career, since professional bodies use the word in formal charges. The light kind is much more common and much vaguer: arriving late, swearing in front of customers, a messy email, arguing with a colleague where clients can hear. Here the word often means little more than "I disapprove", and it is worth being honest with students about that, because it can be used to police clothes, accents and manners as much as actual conduct. It also describes work of poor quality — an unprofessional-looking website, an unprofessional repair job — meaning it looks like an amateur did it. The rare noun means a person without the proper qualifications for a profession; you will almost never meet it.