Chicken out
Definition 1 of 1
Pro Tip 1/3Informal Speech
Used in casual conversations among friends or in informal settings. 
I might chicken out of giving the toast at the party.
Pro Tip 2/3Negative Connotation
Implies weakness or lack of courage, can be mildly insulting or humorous. 
Come on, don't chicken out now, you promised to sing karaoke!
Pro Tip 3/3Common Collocations
Often used with 'of' to specify the action avoided due to fear or nerves. 
She chickened out of making the speech.