Informal
Definition:
To make a major mistake. Completely mismanage or mishandle a situation
Example:
1) Tom screwed up the whole project, and now we're four weeks behind schedule.
Etymology:
This phrase comes from World War II US Army slang.
Definition:
One who makes mistakes regularly; a failure. (Usually written as 'screw-up' when used as a noun.)
Example:
1) Be careful when you're dealing with Charlie -- he's a real screw-up.