Persians were Zorasterians when Arabs attacked Persia during Yazdegerd & Invaded it. Arabs called Persians fire worshipers & infidels , forced them to convert to Islam. Some people converted fearfully because otherwise they would be killed by Arabs. Arabs burned the father/ head of house hold in fire who resisted and did not convert to Islam. They took theeir wives and children as slaves/maidsor sevants. The Persian children grew up and continued working in Arabs’ housholds. There was always this question from any Arab whether the Persian kid is the his kid. The Arab answer was”no” , he is “burned father” meaning he is not my kid , but he is a slave whose fatherwas burned in fire in invasion of Persia.
Source: Dehkhoda Dictionary