When something is encrusted, a hard layer has formed over it and stuck there. Picture a pan left too long: sugar burns and becomes a hard skin you have to scrape. The sea does this to rocks and boat bottoms with salt and tiny shells. Ice can do it to a window. Jewellers also use it: a crown encrusted with stones is so thick with them that they look like a hard covering of sparkle. People stretch the word, too — an old idea can be encrusted with habit, meaning extra stuff has built up around it for years and is hard to chip off. You meet it more in writing than in chat: cooking, nature, fancy objects, and that looser “built-up” sense. The verb behind it is encrust; encrusted is how we usually meet it, as a word that describes the thing after the hard layer is already there.