I've often heard of the phrase "not again" in many American media, however, what does it really mean?

Native speaker’s answer
Rebecca
"Not again" is an exclamation that expresses frustration or anger about something happening "again". The use of the negation "not" emphasises the negative aspect of the situation. You can also use "not again" to emphasise that you don't want to do an action again. Ex: Not again! The power has gone out three times this week. Ex: I burnt the food. Agh. Not again! Ex: We're not watching that show. Not again.