Someone or something that inflicts pain or causes suffering can be described as cruel. It would be cruel of you to offer chocolate to someone on a diet.
High school can be cruel sometimes: cruel teachers, who seem to organize to all pile on the papers and tests for the same day; cruel students, who are mean to each other; and even cruel cafeteria workers, who create disgusting concoctions out of mystery meat. Through its Latin ancestors — crudelis "rude and unfeeling" and crudus "raw or rude" — cruel is related to the English word crude, meaning rude or not fully formed.
