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

I've been meaning to get back into the game after getting bored of, back in game meaning

Pião Mágico em madeira - Fábrika dos Sonhos - Ludolica, pião em ingles americano

💌 on X: “Please end the game and start again” The meaning is, back in game meaning

Panthers place McCaffrey on IR; RB can't return until Week 9 - The, back in game meaning

Pião Mágico em madeira - Fábrika dos Sonhos - Ludolica, pião em ingles americano

I've been meaning to get back into the game after getting bored of, back in game meaning

MDF Personagem P Pintinho Amarelinho Galinha Pintadinha - Dinove Arte e Festa, galinha desenho filme

Back in the Game (TV Series 2013–2014) - IMDb, back in game meaning

💌 on X: “Please end the game and start again” The meaning is, back in game meaning