1. To spend so long in the shower that you have to modify or skip other things you do to get ready for work, school, etc (akin to oversleeping). Signs of this in others include wet hair, mismatched clothing, and/or lack of make-up;
2. To shower so long that you are actually late for an event. Often a catalyst for Murphy's Law to take effect.
"Can we stop at McDonald's on the way into work? I overshowered and I didn't have time to make breakfast."
"Sorry I'm so late! I overshowered and then it took me forever to find my car keys."