It’s a slightly controversial translation form the original Polish “wiedźmin”. Some people translate it instead as “hexer”. The name is supposed to imply that he can do magic, and is also distrusted because of that (think similar connotations to ‘old crone’).
That carries over into the stories. Witchers are somewhat valued for their monster killing abilities, but only grudgingly accepted even then. The rest of the time they’re shunned or persecuted as ‘inhuman freaks’.
