Why did the warden call Stanley a 'caveman' in Holes by Louis Sachar?

The warden called Stanley by his nickname, Caveman.
All of the boys are called by nicknames instead of their real names at Camp Green Lake.  The warden is less interested in the boys’ development than finding buried treasure.  When the warden addresses Stanley by his nickname, which the boys have given him, she is catering to the boys in a way.  She wants them to find her treasure.
Stanley does not even realize that he is Caveman at first.  He is one of the bigger boys.

"The Caveman's one tough dude," said Squid, and he lightly punched Stanley's arm.
Stanley leaned back against the torn vinyl upholstery. Despite his shower, his body still radiated heat. "I wasn't trying to start anything," he said. (Ch. 9) 

He says he doesn’t want to get into a fight with Caveman, because Caveman is kind of a scary nickname.  It isn’t until he realizes that the boys are talking to him and calling him Caveman that he understands that is his nickname.  It sounds tougher than Stanley. 
Stanley decides he likes his nickname.  Having a nickname means that he is one of the guys. It isn’t even that bad of a nickname like Barf Bag.  The nickname means "they accepted him as a member of the group."
When Stanley finally meets the warden, everything about her surprises him.  First of all, he had no idea she was a woman.  She is also meaner to the counselors than the boys, and seems to be more interested in treasure than character-building. 

The Warden turned to Stanley. "Caveman, will you come here, please?"
Stanley was surprised she knew his name. He had never seen her. Until she stepped out of the truck, he didn't even know the Warden was a woman. (Ch. 13) 

When the Warden calls him over to fill his canteen with water, she is making a point to Mr. Pendanski.  This is one of the reasons she used his nickname.  She was being sweet to the boy and mean to Mr. Pendanski.  He contradicted her when he said that he had just filled the canteens, and she did not like being contradicted.  Stanley found the whole incident unsettling.