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catch someone red-handedand catch someone flat-footed
to catch a person in the act of doing something wrong. (See also caught red-handed.) Tom was stealing the car when the police drove by and caught him red-handed. Mary tried to cash a forged check at the bank, and the teller caught her red-handed.
catch somebody red-handed
to discover someone doing something illegal or wrong (often + doing sth) I caught him red-handed trying to break into my car.
Also, catch in the act. Apprehend someone in the course of wrongdoing, as in The boys were trying to steal a car and the police caught them red-handed, or He tried to cheat on the exam, but his teacher walked in and caught him in the act. The first term referred to blood on a murderer's hands and originally signified only that crime. Later it was extended to any offense. The variant ( catch in the act) is a translation of the Latin in flagrante delicto, part of the Roman code and long used in English law.