These terms mean "open," "sincere," "without reservation," "straightforward." Thus we may speak of a "frank statement" or "a candid reply," meaning something that is without disguise, pretense, or reserve. Candid, derived from a Latin term meaning "to glow," is less blunt in meaning than frank; a candid remark is less outspoken and perhaps more tactful than a frank one, but the meanings of these terms are normally interchangeable. Approximate synonyms include open, unrestrained, uninhibited, and outspoken.
Dictionary of problem words and expressions. Harry Shaw. 1975.