Stealthy describes a person, an animal, or a movement that is quiet and secret — done in a way designed not to be seen or heard. A stealthy cat. A stealthy step. A stealthy look at someone's phone screen. The key idea is not only that it is quiet, but that the quietness is deliberate. Someone is trying not to be noticed, and usually has a reason for it. Because of that reason, the word often has a faint smell of something being hidden. A stealthy movement makes a reader slightly suspicious — what is this person up to? But it is not always bad. Soldiers, hunters, and nurses on a night shift all move stealthily for perfectly good reasons, and 'stealth aircraft' — planes built to be almost invisible to radar — use the same idea proudly. It is a slightly literary word, more at home in stories and news writing than in daily talk. In conversation most people would say 'quiet' or 'sneaky'. 'Sneaky' is close in meaning but feels more childish and more clearly disapproving — a sneaky child hides a biscuit; a stealthy soldier crosses a field. Stealthy sounds skilful; sneaky sounds naughty. Two forms to keep straight: stealthy is the describing word (a stealthy cat), stealthily is the how-word (the cat moved stealthily), and stealth is the thing itself (he crossed the room with great stealth).