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let (something) drop
1. To cease discussing or dwelling upon something. Look, we can't afford a new car right now, so just let the matter drop already!
2. To accidentally or inadvertently reveal some secret or important piece of information. I can't believe your sister let news of our engagement drop in front of your parents last night! The agent, under the influence of a powerful truth serum, let drop the names of other operatives working covertly in the area.
Also, let fall. Utter a word or hint, either casually or inadvertently. For example, He let drop the fact that he'd decided to run for office, or She let fall some bits of gossip about the other teachers. [Late 1500s]