To have a tendency to be cowardly or easily frightened. She likes to strut around like she's the boss of the place, but she has a yellow streak down her back if you ever confront her on an issue.He knew what they were doing was wrong, but he has a yellow streak down his back that kept him from intervening.
have a yellow streak down one’s back
tv. to be cowardly. (Have got can replace have.) wrong. If you have a yellow streak down your back, you don’t take many risks.