The dictionary recognized word “low-key” is an adjective that means, according to Merriam Webster:

1: quiet and relaxed: not very forceful, emotional or noticeable

2: of low intensity: restrained

The slang variant of low-key (often written without the hyphen as lowkey) functions as an adverb. Lowkey is typically used to describe a speaker's desires or emotions. Lowkey retains the dictionary definition’s meaning of “of low intensity” and “not very emotional.” However, additionally, it can also indicate something that is secretly (perhaps somewhat shamefully) wanted or felt by the speaker.

In short, lowkey is used to describe a speaker's thoughts, feelings, and desires, and it means:

1: moderately: of low emotional intensity
2: secretly

The opposite of lowkey is highkey.

1. quiet, modest, not very assertive

2. to treat with little emphasis

When someone wants to do or be something but keeps it on the low for people not to find out

To keep something a secret; to not announce something.

To be secretive about something Such as crushes,events,Etc.

The opposite of high key. Basically just not very showy. If someone thinks you are low key kinda hot, it means that they find you quite attractive but you are very subtle about being attractive. You almost don't know it.