What is the etymology of the term ‘Gish’?

I understand that it roughly means a fighter/mage, as per this question: How to optimize a Gishy [Fighter/Mage] Character?

But I just wanted to know more about how the term came about. It’s so hard to keep up with the kids these days, learning the lingo helps.

Is it related to Githzerai in DnD? A good answer would include the root words, and a few examples from RPG forums with context (if they exist; I could only find this).