over (one's) head
1. Too complicated to be understood by one. My jokes always seem to go over Stephanie's head, so I'm glad you think I'm funny at least. A lot of this technical stuff is way over my head.
2. Bypassing one's authority. She just wouldn't listen to me, so I had to go over her head to her boss and complain about it.
3. Lingering as a source of concern or worry for one. All these bills are hanging over my head, and I don't get paid for another week. I'm so glad that exam is over—it's been over my head for months.