The phrase "on the Q.T." means to do something in secret or in a confidential manner. It is often used in casual conversations or in writing. There are many synonyms for the phrase such as covertly, clandestinely, furtively, surreptitiously, secretly, privately, discreetly, and confidentially. Each of these synonyms has a slightly different connotation, but they all imply secrecy or confidentiality. When using these words, it's important to consider the context and the audience to ensure that the intended meaning is understood. Whether in personal or professional settings, using these synonyms can add variety, nuance, and flavor to the language used.