A phrase entering greater popularity is man up, an idiom urging the other party to behave in a less submissive manner.
Example: The best example is from the recent debate between Senate Majority Leader Harry Reid and challenger Sharon Angle, where Angle urged Reid to "man up" and face the United States' long-term financial issues. This implies that Senator Reid was hiding from these problems rather than face them.
The implication, particularly if the target of the idiom is a man, is that the target has been behaving in an "unmanly," cowardly, timid, "chicken" manner. Unsurprisingly, this implication is considered insulting.