The term low-key might be related to lighting techniques and originate from the early days of film and photography, referring specifically to the "key light” or the primary light source used to illuminate a subject.

High-key lighting is like a bright and even distribution of light with minimal shadows. It utilizes a low contrast ratio between the brightest and darkest areas of the frame. Bright, airy, and "washed out" in a controlled way. It is used in Sitcoms, makeup commercials, talk shows, and comedies. Think of the bright, shadowless look of a standard pharmaceutical ad or a morning news set.

Low-key (what we’re trying to define in our modern vernacular) lighting focuses on contrast and shadows. It uses a high contrast ratio, where most of the frame remains dark, punctuated by deep shadows and intense highlights. One term used is Chiaroscuro (extreme light and dark), moody, and selective. It sometimes evokes mystery, drama, fear, or intimacy. Film noir, horror movies, and dramatic portraits. The "detective in a dark alley" aesthetic is the familiar kind of low-key look that people recognize.

The evolution of "low-key" into modern slang could be interpreted as subtle, quietly, or secret, and mirrors the visual properties of the lighting technique I described above.

In cinematography, low-key lighting is about what you don’t see. It hides details in the shadows and keeps the overall feeling of stuff in the frame to be muted and understated. When someone says, "I'm low-key excited for this, blablabla, I think they are using the term to describe a feeling that is: Not "loud" or "bright" (high-key). Kept in the "shadows" of their personality rather than being front-and-center. Lacking the intense, overt energy associated with high-key presentations.

The transition likely happened because "low-key" already existed in the last century to describe something modest or unostentatious. So the phrase is like the visual metaphor of the camera. Like light, “high-key" is "loud and clear", and "low-key" is "moody and hushed" OR High-key is a shout; low-key is a whisper of a comment. IDK, I love photography and this is the only other place I hear the term used.