There are multiple origins and meanings for the name Alexandra:
1. Ancient Greek name (Ἀλεξάνδρα), derived from the ancient Greek words (αλεξω) [alex] - "defender, protector" + (άνδρ) [ander] - "men and women, people" meaning - "she-protector of the mankind". The female equivalent of the ancient Greek male name Alexander.
2. Derived from the Greek name Cassandra - "shining". The names Alexandra & Cassandra recorded in the Mycenaean texts (the monument of the ancient Greek dialect writing (16th to 12th centuries BC)
Saintt Alexandra was the wife of the wicked Emperor Diocletian and was a Christian, but secretly. While Diocletian was having St. George tortured, the Empress went to the arena and bowed before St. George and professed her faith openly. Her husband was so outraged by this and by so many people being converted by witnessing George's miracles and patient endurance, that he ordered both of them to be beheaded. Alexandra quietly accepted her sentence and prayed as the guards walked her to the place of execution. She asked if she could rest for a moment. The guards allowed this. While resting by the wall of a building, she quietly gave her soul to God. She reposed on April 21, 303. Two days later, St. George was beheaded. They are commemorated at the same time along with Anatolios, Protoleon and the 630 others who were martyred for professing faith while witnessing George's martyrdom.