Wimp out
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
Used mainly in casual conversation rather than formal speech or writing. 
I nearly wimped out on the bungee jump, but my friends cheered me on.
Pro Tip 2/3Negative Connotation
Implies a negative judgment, suggesting weakness or lack of determination. 
He wanted to try the spicy challenge, but he wimped out.
Pro Tip 3/3Emotional Implication
Can imply anxiety or fear, not just laziness or lack of interest. 
Even though she loves singing, she sometimes wimps out before karaoke.