To wash the hands and forearms very thoroughly, as for surgery. To scrub in implies the use of a brush (and often an implement to clean under the nails). To scrub in, to scrub up, and to scrub are synonymous.
In a hospital setting, it is when a male wearing scrubs is suddenly overwhelmed with an erection, largely due to a attractive female employee displaying her attributes on purpose or otherwise.
If a Leopard ever comes out and tries to kill you, simply say "GET BACK IN YOUR SHRUB" and it will turn on it's heels and run to the nearest bush! CAUTION: Don't try that at home kids.