Here's oxford dictionaries on the word: http://www.oxforddictionaries.com/definition/english/ask

[WITH ADJECTIVE] informal A demand or situation that requires a specified degree of effort or commitment:

it is a big ask for him to go and play 90 minutes

it was a tough ask, but they delivered

Or OED: http://www.oed.com/view/Entry/11505?rskey=xAEeZD&result=1#ei...

Westm. Papers 1 Nov. 107 Who has not suffered when he has played correctly second hand..from his partner assuming that there has been an ask for trumps?

2014 D. Chapman Ascendant lxxiii. 301 A plane? Full of people?.. That is an enormous ask.