Русско-англо-русский юридический словарь

In flagrante delicto is a Latin phrase which means “in the very act of committing an offense.” The term in flagrante delicto is used to indicate that a criminal has been caught in the act of committing an offense. The Latin term is sometimes used colloquially as a euphemism for a couple being caught in the act of sexual intercourse. The colloquial "caught red-handed" or "caught rapid" are English equivalents of this Latin phrase. (USLegal)

catch red-handed. 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. (The American Heritage® Dictionary of Idioms by Christine Ammer)