What is the meaning of the phrase "spoiler alert"?
"spoiler" is clear but why does it go together with "alert" in the context below. Penny should have mentioned earlier the alert of spoiling.
"Penny: You-you know how in Harry Potter and the Half-Blood Prince, Ron didn't abandon Harry just because Harry started dating Ron's sister?
Leonard: Harry and Ginny get together?
Penny: Sorry. Spoiler alert." (The Big Bang Theory)

Question

Answer

A "spoiler alert" is a warning that important parts of the movie (ie. parts that are meant to be a surprise) are or will be mentioned.  The meaning is, "ignore what I just said" or "cover your ears - I'm about to tell you the plot".

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