A good way I think of to define humility is as the inverse of your willingness to argue with future you. Imagine that yourself from a few weeks in the future (or 5 years in Matthew McConaughey's case) steps out of a time machine. Would you be willing to concede that he knows more?

Examples:

• The student who is certain of his answer will expect that it will not change, so he is not humble at all about it.
• The student who is resigned to the fact that the answer is unknowable expects that future her doesn't know any better so she's not humble either.
• The student who rechecks her answer anticipates that future her found a mistake, otherwise she wouldn't bother checking. That's how you know she's humble.
• I'm humble about my assessments of the probability of creating an AGI. I would immediately take future-me's word on it because he will surely know more.
• I'm not humble about my belief in MWI, because I don't expect that future me will know more about it. The only thing that could change my mind is an experiment disproving superposition for cat-sized objects, which I don't expect me-in-5-years to see. If future-me doesn't believe in MWI I would need to hear all of his arguments, I wouldn't agree with him on the spot (maybe I'm going to get hit on the head in two years?)

I believe that people systematically underestimate the amount that the world, themselves and their opinions will change in 5 years. That would amount to a bias for under-humility.