have a yellow streak down one's back
have a yellow streak down (one's) back
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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.