Cloak and dagger
Describes any covert operation involving intrigue or secrecy; Chaucer in Canterbury Tales, The Knight’s Tale c. 1387 makes mention of “the smiler with knife under the cloak”, which does smack of intrigue but this just might be coincidental. During the 17th and 18th centuries a whole genre of French and Spanish melodramas were called de cape et d’épée and de capa y espada (cloak and sword) in French and Spanish respectively. It was not until the early 19th century that the exact phrase cloak and dagger appeared in Barnaby Rudge (1841) when Dickens made a sarcastic comment about these melodramas. Only one year before Dickens, Longfellow referred to the genre as cloak and sword, a literal translation from the French and Spanish. Certainly, by the mid-19th century the expression was being used as it is used today.