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.