let (something) fall
To accidentally or inadvertently reveal some secret or important piece of information. I can't believe your sister let news of our engagement fall in front of your parents last night! The agent, under the influence of a powerful truth serum, let fall the names of other operatives working covertly in the area.
ˌlet ˈfall something
mention something in a conversation, by accident or as if by accident: She let fall a further heavy hint.