Spoiler

Spoilers were things one individual knew about another's personal future. River Song particularly favoured the term, and used it to warn the Tenth and Eleventh Doctors when they asked questions that she couldn't answer without revealing details about their futures. (TV: Silence in the Library, The Name of the Doctor)

The Tenth Doctor first heard the term from River when they met in The Library, but River first heard the term from the Eleventh Doctor in Hitler's Germany. (TV: Silence in the Library, Let's Kill Hitler)

Behind the scenes
Steven Moffat included the reference as a slight dig at fans who wanted to know details of forthcoming episodes of Doctor Who before they aired. (CON: Shadow Play)

ספוילר Spoiler