1. (verb): When someone attempts to steal the limelight or credit for something, under the pretense of "helping", but really makes himself just look bad in the process.
2. (noun/verb): Inadvertent hijack
It is often meant to help, but through some devious alterior motive or actual impact over intent, it ruins the moment. People who do this are usually associated with santorum.
Yo, did you see what Kanye did at the Grammy's? Yeah, he totally co-jacked Taylor's moment.
In an effort to help Frank's presentation, Lizzie co-jacked him, taking all the credit when all she wanted to do was make sure the boss knew she helped too.