An unprincipled person is someone who lacks moral principles and behaves in an unethical or dishonest manner. A ROGUE is someone who behaves in a dishonest or immoral way, often in a devious manner. A REPROBATE is someone who has been rejected by society due to their immoral behavior. Finally, a KNAVE is someone who is untrustworthy and deceitful. All of these terms can be used to describe an unprincipled person.