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Crooks is actually a nickname. We don't know his real name. This point alone is worth pondering. The other person who is nameless is Curley's wife. Namelessness in Of Mice and Men shows the extent of the alienation that is embedded on the ranch. In Steinbeck's novella, the world is a brutal place and alienation and separation is the norm. 

In light of this, it is no wonder that Crooks and Curely's wife have no names. They barely exist. 

Now as for why Crooks is called Crooks.

In a conversation between George and Candy, Candy explains what life is like on the ranch. When they come to the topic of Crooks, Candy say that Crooks is called Crooks, because he has a crooked back. According to Candy, a horse kicked him. Here is what Candy says:

“Yeah. Nice fella too. Got a crooked back where a horse kicked him. The boss gives him hell when he’s mad. But the stable buck don’t give a damn about that. He reads a lot. Got books in his room.”