A deceitful person tells lies, or hides the truth, in order to get what they want. We use it for people, and also for smiles, words, promises, or ads that look friendly but are not honest. It is stronger than not honest, and it judges character more than one mistake: if you call someone deceitful, you are saying this is how they operate, not only that they told one lie. Though actually, a thing can be deceitful too — deceitful packaging, a deceitful appearance — when it is designed to mislead. It is not a word to throw at a friend lightly.