--something that occurs to you, often deep and profound--that occurs to you while you're in the shower.
Many people, for some reason, do their best thinking in the shower. Maybe it's the quiet, distraction-free environment, or maybe it's the tingling feeling of water rushing over you that stimulates your senses and mind, but some of the best and most creative mind-work is often done at the least-convenient moment (i.e. when shampoo suds are falling into your eyes).
Showerpiphanies are often ephemeral, since (unless you have something waterproof to write with) you can't note them down, and once you get out of the shower and busy with other things, they're easy to forget.
, when something that needs to be done occurs to you while you're in the shower, which you then forget as soon as you get out.