To study the etymology of a word is to study its history and its origin.
The etymology of the word jealous shows us this:
“Most of the words for ‘envy’ … had from the outset a hostile force, based on ‘look at’ (with malice), ‘not love,’ etc. Conversely, most of those which became distinctive terms for ‘jealousy’ were originally used also in a good sense, zeal.
So jealous has its roots in Greek in the word zeal. A God who is zealous for me is passionate and committed to me and my good. A far different spin than a jealous God who is based out of malice.
A great web site to study the etymology of words is: Online Etymology Dictionary
