When someone is beyond angry. Full of rage. Hornets are really angry and relentless, hence the term, "mad as a hornet".
1. Jane was playing hide and seek and was trying to hide in the closet quickly, but smashed her finger in the closet door. Jane was mad as a hornet.

2. If someone is very upset, you ask them "are you mad as a hornet?" Their reaction will tell you how mad they are.
by Marie G the DiscoDiva October 11, 2006
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