The character was originally shaped like the profile of a standing person, with the eyebrows and eyes standing out. From the current version of “mei”, you can still see one eye and one eyebrow.
In Chinese, there's a phrase called "wuguan" (five sense organs), which refers to ears, eyebrows, eyes, nose and mouth. Eyebrows form the first line of defense for the eyes. They also enhance the beauty of the face. The make-up of brows meant a great deal to ancient Chinese women and drawing eyebrows was a "must-learn lesson" for them.