It's pretty overwhelming when you ask your friend a seemingly innocuous question, like "Do you like hot dogs?" and she unleashes a diatribe about the evils of eating meat. A diatribe is an angry, critical speech.
This noun has its roots in the Greek diatribē, "pastime or lecture," from diatrībein, "to waste time or wear away," combining dia-, "thoroughly," and trībein, "to rub." So the origin of the word diatribe is connected to both serious study and the spending or wasting of time. With most diatribes, the speaker thinks he's well informed and knows something the listener doesn't, while to most listeners the diatribe is so angry and unhinged that it's just a waste of time.