meaning - What's a yellow line running down somebody's back

The only line between good and evil is the yellow one running down your back. What line does it speak of? I'm also not sure what meaning does "run down" have in this context. The source of

Yellow Poop: What Does It Mean and Should I Be Worried?

This Means War! For Boys Only: Dyan, Penelope, Dyan, Penelope: 9781935118657: : Books

How can Someone Recognize DISC Styles in Others?

United States Pavement Markings: Two-Way Traffic Markings Long Descriptions - FHWA MUTCD

5 Dollar Traffic Schools

Section 6: Navigating the Roads - California DMV

Estoppel

Did you have changed your email Facebook account before you got locked ? : r/facebook

Understanding Pavement Markings Safe2Drive - Driver Resource Center

United States Pavement Markings: Two-Way Traffic Markings Long Descriptions - FHWA MUTCD

Fiction: Fast Lines - The Surfers Journal

What is the meaning of the solid yellow line on roads? - Quora

Brown Line on Nails: Vitamin Deficiencies and Other Causes

Matthew Cochrane on X: The labor market is slowly shifting away from looking at one's resume (schools, previous jobs) toward matching skill sets. When you look at skills, it's kind of the