WIMP OUT OF

wimp out (of (sth))

(CHICKEN OUT OUT SOMETHING)

to get out of something, leavingothers to carry the burden. : Come on! Don't wimp out nowthat there's all this work to be done.

http://dictionary.reference.com/browse

e.g.

You lost the bet and were supposed to paint your face with green paint for a week. After a couple of hours, you already wimped out of it.

or

I order 'sissy' pizzas, wimp out of football and found Movember an ordeal. How can I be more manly?

http://www.guardian.co.uk/commentisfree/2011/dec/04/want-to-be-more-manly