1. a difficult or demanding request
I know it's a big ask, but...
If something is considered to be 'a big ask', it's likely to be something you expect to be difficult to do, either for the person you are asking or in general. If your friend was terrified of driving, for example, it would be 'a big ask' to expect them to drive you to work if you were suddenly unable to do it yourself. Generally, you present someone with 'a big ask' (or a difficult or upsetting task) when you have very few options; and so you are likely to use it when the person who might not want to help is the only person capable of doing so.