Ooh, well, my book's title is Stillwaters, and I'll try to explain this as spoiler-free as possible.
So Stillwaters, still and waters combined. My MC has an extreme fear of deep, vast bodies of water, to the point that he doesn't dare stand too close to them. He has amnesia, so he doesn't remember the source of his trauma, but this is just a bit that has a strong connection to the title.
This was a note I made:
The title is derived from the aspect of calm waters suggesting calm and silence, but as the title suggests, ‘still’ suggests something eerie, like a sleeping monster. This means that beneath its peace, something is hidden, most probably something dark/twisted. And like the fragility of water, one little tap can break the stillness of its surface, same as the facade of many things.
And considering my book is majorly about rich teenagers flawless on the outside, while being pretty damn rotten underneath all their glamour, I'd say the last sentence is rather fitting.