What is the origin of aegis?

Aegis “protection, support” comes by way of Latin from Ancient Greek aigís “shield of Zeus or Athena,” which may derive from aíx (stem aig-) “goat” in reference to a type of cloak or shield made of goatskin, plus -is, a noun-forming suffix. Because the stem of aigís is aigíd-, aegis may be pluralized in English either as aegises or, more traditionally, as aegides. Ancient Greek aíx does not have any clear relatives in modern English, and it remains uncertain whether aíx is of Indo-European origin or is a borrowing from a Middle Eastern source. Be sure not to confuse aíx with Latin agnus “lamb,” which looks similar but is not related. Aegis was first recorded in the mid-15th century.