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.
See also: back, down, have, streak, yellow
Farlex Dictionary of Idioms.

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.
See also: back, down, have, streak, yellow
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • a yellow streak
  • appear to
  • a change of heart
  • a tough cookie
  • a tough customer/cookie
  • a mystery to (one)
  • a turn of phrase
  • able to do
  • able to do it
  • a shoulder to cry on