A phrase that’s always bugged me, and is a perfect illustration of the way clichéd phrases function by sanding down the meanings of their component words: “under lock and key.”
That makes no sense. Yes, if you want to keep something safe and secure, putting it “under lock” is a good idea…but if you include the key, the lock might as well not be there. It’s rather curious to imagine how the phrase originated: “under lock” just didn’t seem good enough? We needed a couple of extra syllables? A “key” does not aid security; it aids access to whatever is hidden or protected by the lock.