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

What Does a Double Solid Yellow Line Mean?

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

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

Political brouhaha erupts in Antrim County over dock installation request by Gov.'s husband, Local News

The yellow and white lines on the road have a message. Don't ignore them

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Estoppel

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