When you are afraid of something, but you don't know what it is. Used in the novel and movie, Breakfast At Tiffanys.
...the blues are because you're getting fat or because it's been raining too long. You're just sad, that's all. The mean reds are horrible. Suddenly you're afraid and you don't know what you're afraid of.
Period, as in a girl's period.
Guy 1: So how's your sex life?
Guy 2: No good, my missus has the mean reds...