New York Times (August 6) reported under the title, “25 years after Gardner Museum heist, video raises questions” that Federal officials released a video tape recording a guardsman, Richard Abath’s actions on his duty at the Isabella Stewart Gardner Museum in the morning of March 18, 1990, the day 13 artworks including three Rembrandts and a Vermeer were stolen from the Museum. It reads:
In 1990, Richard Abath, a 23-year-old who, while moonlighting as a security guard, allowed two thieves posing as police officers to enter the Isabella Stewart Gardner Museum here. For 25 years, his decision to buzz in the men who carted away $500 million worth of art has been largely treated as the hapless act of a rookie watchman who fell for a bluff.
I can understand “fall from a bluff,” but I don’t know what “fall for a bluff” means. By Googling, I barely found “fell for a bluff” in the Sport Rack's article (March 15, 2015):
“The Stormers finally surrendered their unbeaten record when they were defeated 28-19 by the visiting New Zealand side, the Chiefs, in the Super Rugby match at Newlands on Saturday afternoon. After the match, Stormers captain Duane Vermeulen said his side fell for a Chiefs bluff.”
I don’t find “fall for a bluff” as an idiom in English dictionaries at hand, nor its currency on Google Ngram. What does it mean? Is this a popular English idiom?