All of the people in "Of mice and men" are outsiders in there own right, they have all been disallowed, or to a certain extent not wanted to have certain rights because of things such a sex, race and different disabilities. The word 'outsider' means to not be in a certain group of people (out side of a group), it can also be linked with out casts who are people thrown away from the "normal" group because of some thing they normally had no control over.
Crooks is a black man in the 1930s, this was especially not a nice time to be a black person. In America at this time there was an awful of prejudice agents black people, they had been slaves not that long ago and some people still thought that this was the right way. For example the white supremacy group the KKK, would go around and lynch (hang) black men for looking at a white woman (this would get turned into that he had raped her). The equal rights protestors of the 60s where a long way off and black people had very few rights. This point is especially put across in the quote by Curley's wife when Crooks talks to her in a objective way on page 85 "Well, you keep your place then, Nigger. I could get you strung up in a tree so fast it ain't even funny." The language used to talk to Crooks for example "Nigger", is offensive; it also labels him in to a different group from them making him an outsider.
Curley's wife is an outsider because she is a woman; there are no other women on the ranch so she is an outsider. A woman is different to a man in the way they live and want to be respected, there feelings can be easily hurt and the men do not seen to understand or care.
Candy is an outsider because he is old and crippled, and when your are old and or crippled you are useless, your employer will not want to employ you so you are gotten rid of, this is show in the part of the book where they shoot Candy's dog because of the same reasons.