The frontal secretions left on a woman's pair of panties. A womans genital moistness on her panties. When the vagina releases it's juices onto a pair of panties and leaves a stain, a vertical stain.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.