The term "letting upon" does not have any direct antonyms as it is not a commonly used phrase. However, if we analyze its meaning, we can find some related antonyms. The phrase can be understood as revealing or disclosing something that was previously kept secret or hidden. So, the antonyms would be words that mean concealing, hiding, or keeping silent. Some of the antonyms for letting upon are covering up, denying, keeping back, obscuring, suppressing, and withholding. These words suggest a sense of secrecy and suggest that something is being kept from public knowledge. In short, there are no direct antonyms for "letting upon," but we can still identify related words that can be considered as its antonyms.