Shaking in one's boots

Shaking in one's boots is an idiom which means a person is so frightened of something that he or she is physically shaking, in such an extreme manner that the shaking is visible through the person's boots.

It should be noted that this idiom, as commonly used, is almost always sarcastic, and that the person is not actually frightened at all. It usually refers to a threat from another person, and means that the speaker believes that the threat is empty. One might say "You've got me shaking in my boots over that", meaning that one does not take the threat at all seriously, and is in fact not the least bit frightened by it.