chicken out

verb

1. informal.
   intr, adverb to fail to do something through fear or lack of conviction

Idioms and Phrases

Back out from fear, lose one's nerve, as in In the end I chickened out and took the easier route down the mountain . Chicken is a popular synonym for “cowardly,” a usage arising in the 1600s and 1700s but then apparently abandoned until the 20th century. [ Slang ; c. 1930]