v. To act in accordance with established masculine roles in a given situation, implying courage and responsibility. Usually used in the imperative, the term is a favorite of police who want suspects to incriminate themselves. Ironically, advice to 'man-up' is usually followed by a promise that it is the least painful alternative.
You signed for a package full of weed, now you need to man-up, and admit that you knew what was in it. It'll go easier for you with the judge.