1
general::
Phrase(s): chicken out (of something)
Inf. to manage to get out of something, usually because of fear or cowardice. • Come on! Don’t chicken out now! • Freddy chickened out of the plan at the last minute.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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]
American Heritage Idioms