Low-key (two words, I think with a hyphen, not one word) is a very colloquial word that kind of means “toned down”, or “relaxed” or easy going. I think it’s probably more a US English word (I’m a native US speaker myself) than one used in the UK. I’ll defer to my UK friends on that. Plus, I’d say the word or phrase is very context driven with regard to its meaning. For example, you might say “I’m a low-keyed kind of guy (I am by the way).” That means I’m pretty easy going, easy to get along with, I don’t get riled up easily, that sort of thing. I think the word tends to be used with people rather than things, because I can’t think of an example where you’d use it to describe a thing. You might say “the song is rather low-keyed” but I can’t say I’ve heard that all that much. Maybe. And the word normally appears as “low-keyed” rather than just “low-key” by itself. Hope that helps.

Lowkey is slang and it means to keeps discreet , having a low profile, not announced. For example would be " he lowkey doesn't talk to anyone " which can mean " he is known to keep to himself " or " she lowkey likes that boy " which could mean she likes the boy a little bit like saying she has a crush on him. It all depends I hoped this helped.

Just a big thanks to Gary for his clarification. Yes, low-key seems like it should be okay, but no one here (California) ever says it that way. It is used frequently, but always as “low-keyed.” It’s just a regional difference apparently. Almost every time I’ve ever heard it used it referred to people, but for some reason I kept thinking I must have heard it used without reference to a person. It’s kind of rare, however, if this occurs. I guess you could say “it was a low-keyed movie” or something, as opposed to an action-packed one, but I doubt I’d ever say that myself. If someone else did it would be easy enough to understand, but it would sound a bit odd to hear for some reason.

It means plain, without a lot of extra stuff. If one of your friends is having a birthday - you might throw him a party. You could rent a location, pay for catering, invite 50 of his closest friends, hire a band, buy him a huge gift, etc. etc. etc. Or you could keep the party "low key" where you and 2 or 3 mutual friends meet at a restaurant and you buy him dinner.