Knee-jerk reaction

A 'knee-jerk reaction' is the reaction you have to something without giving it much or any thought.

Example:
-Why did you swerve into the other lane of traffic?
-I had a knee-jerk reaction. I saw the deer and didn't want to hit it!

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!