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
The opposite of lowkey is highkey