What is the figure speech for contraband?

The term "contraband" itself is not a figure of speech; it refers to goods that are illegal to trade or transport. However, in a metaphorical sense, it can be used to describe something that is prohibited or forbidden, evoking a sense of secrecy or illicitness. For example, one might refer to "contraband thoughts" to suggest ideas that are socially or morally unacceptable. In this context, it implies a hidden, rebellious nature associated with the concept of contraband.