A state of being where one is immersed extraordinarily deeply into a project or situation.
Originating from farm life, where cattle and horse breeding often requires literally going "shoulder-deep" into an animal to either fertilize its eggs manually or to check for a pregnancy. This is only gross the first dozen times. Ask your veterinarian or local hick for further details.
Man, I have to get my nerd on and start coding this back end content managed system of this web site my damn self. I'll be shoulder-deep in figuring it out.