1. Modest and restrained, often intentionally to avoid being conspicuous, loud, wild, etc. It's just going to be a little, low-key gathering with friends, no big deal. I'm trying to keep news of my promotion low-key, so as not to upset some of my co-workers.

2. Not exhibiting much energy or enthusiasm; mellow, subdued, or melancholy. You seem pretty low-key today. Everything OK?

3. Describing something that is done or felt in a subtle way. I'm low-key failing this class and don't want anyone to know. I'm kind of low-key in love with him.