Someone who is apoplectic is not just mad — they're so filled with rage, they can barely communicate. If your mother is apoplectic, you'd better hope it isn't over something you did.
Apoplectic comes from a Greek word that means to "disable by a stroke." What is a stroke? The sudden loss of consciousness or control caused when a blood vessel in the brain ruptures or becomes blocked. When this happens, a person becomes apoplectic. This word also applies to someone who acts so upset, it is like he or she is having a stroke.