If something is "above" your understanding, it is too difficult for you to understand. The image is of something just out of your reach, over your head. You try to understand it, but you cannot.
Cleaning ones mind of misunderstanding
Keep an idea in ones head to act on it later.
If you have your hands full, you can't do anything else with them, can you? This idiom means that you're overloaded and don't have time or ability to do anything else.
The term "nest egg" refers to ones financial savings for retirement.
Er ... what? Are you asking what "under one's wing" means? Here's a link.
If I do something "behind your back", it means that I do it without your knowledge, usually in a sneaky way.
That the referenced item is important. When sorting potatoes for sale, the smallest ones are typically tossed aside as being not good enough to sell.
It's not really an idiom - if something is at your heels, it's following close behind you. This means right behind you, almost upon you.
To take a risk, or make a sacrifice on another's behalf. Ex. "I stuck my neck out for you when I told the boss that I was the one who made your mess."
To lose ones temper and react violently.
eight mr. patatos
To "dig ones grave" is to get ones self in trouble. Example- He dug his own grave when he decided to steal from the teacher and then brag to other faculty about it.