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

The Shifting Symbolism of the Gadsden Flag

As an exercise, I divided (publicly available) excerpts from Mistborn, Ulysses, and Sea of Tranquility. Green is scenery, red is characterization, blue is plot, and yellow is exposition. In the frequent case

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Meaning of Yellow Lines by CunninLynguists (Ft. Phonte & Witchdoctor)

What Does a Double Solid Yellow Line Mean?

In the US, what is the difference between yellow and white lines on the road? - Quora

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

All Emoji Names & Meanings - Trusted Since 1922

Rob Freund on X: Yesterday, a court ruled that this🌝emoji could constitute securities fraud: / X

Walmart Jefferson

IUD insertion: A guide and what to expect

Estoppel

55+ Popular Emojis and Their Meaning - Parade

Various Artists: The Legendary Studio One Records (2LP)

Yellow Tongue: Causes and How To Get Rid of It