when was the above mentioned term first used?
It was in a letter to Akhenaten that is addressed to "Great House, L, W, H, the Lord"
Pharaoh
Sometime during the era of the New Kingdom, Second Intermediate Period, "pharaoh" became the form of address for a person who was king. The earliest confirmed instance where "pr ꜥꜣ" is used specifically to address the ruler is in a letter to Akhenaten (reigned c. 1353–1336 BCE) that is addressed to "Great House, L, W, H, the Lord". 
egyptian kings were believed to combine divine and mortal qualities UNANSWERABLE name a few egyptian god's. Sekhmet, Amun, Ra, Nekhbet, Nut, Geb to name a few and who was the head of the egyptian people? UNANSWERABLE what is the meaning of the first egyptian god mentioned above? She is a warrior goddess as well as goddess of healing is she the daughter of any of the above mentioned god's? Yes, sometimes called the daughter of Ra and was there a temple built for the above mentioned goddess? Note was made of a temple for Maahes that was an auxiliary facility to a large temple to Sekhmet at Taremu in the Delta region who discovered this? UNANSWERABLE in egyptian myth, who is known as human son og the god's? Pharaoh
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Pharaoh [SEP] Etymology
The earliest confirmed instance where "pr ꜥꜣ" is used specifically to address the ruler is in a letter to Akhenaten (reigned c. 1353–1336 BCE) that is addressed to "Great House, L, W, H, the Lord". However, there is a possibility that the title "pr ꜥꜣ" was applied to Thutmose III (c. 1479–1425 BCE), depending on whether an inscription on the Temple of Armant can be confirmed to refer to that king. During the Eighteenth dynasty (sixteenth to fourteenth centuries BCE) the title pharaoh was employed as a reverential designation of the ruler. About the late Twenty-first Dynasty (tenth century BCE), however, instead of being used alone as before, it began to be added to the other titles before the ruler's name, and from the Twenty-Fifth Dynasty (eighth to seventh centuries BCE) it was, at least in ordinary usage, the only epithet prefixed to the royal appellative.