Afraid of your own shadow

e.g. My dog was afraid of its own shadow when it was a puppy, but now it barks fearlessly at the biggest dogs.

Meaning: very easily frightened, especially of small or imaginary things

Oringin: Shadows can be scary. Horror movies, for instance, are full of fearsome shadows. They sometimes make you hold your breath and jump out of your seat in terror. No wonder that umbra, the ancient Latin word for "shadow," also means "ghost." There are real things in this world to be fearful of, but if someone is afraid of his own shadow, then he 's a very jumpy, nervous person who panics easily.

指非常胆小。拉丁语中，umbra一词既指影子，又指鬼魂。