It is a Spanish verb that is used for the case (very common today in social networks) in which an important fact or situation is revealed that ruins the plot in a book, movie, series or story in general to those who are following them. Although much less in Spanish, it is also used in political campaigns when a candidate with little voting intention harms another with a higher chance of being elected. It comes from to spoil ("going after something or someone to modify or harm your situation").
