To legitimize something, especially something bad, means to officially allow it, approve it, or make it seem acceptable.
例句
They will accept no agreement that legitimizes the ethnic division of the country...他們不會接受任何使國家民族分裂合法化的協議。Images which glorify violence and cruelty, serve to legitimise such behaviour.這些美化暴力和暴行的圖片，就是要使此等行為合理化。