Knee deep in

'Knee deep in' is used to describe when someone is in the middle of doing something and doesn't want to or can't be interrupted. The visual it suggests is that the person is standing in something that is up to their knees, and they can't easily get out, if out at all. It can be used as an apology.

Example:
-I'm sorry I can't go to lunch with you! I'm knee-deep in paperwork!
-Maybe tomorrow!