Lucas is frequently given credit for coining the word "greeblies" but as the articles
DamnYankee123 posted document, the term (and the synonyms "nurnies" or "wiggits") was already in common use well before production of Star Wars began. I will note that Star Wars is unquestionably responsible for the term coming into widespread usage outside film production teams - I know my own first encounter with "greeblies" was in one of the original trilogy sketchbooks which published concept design sketches of the various models and props used in the films. Those drawings included occasional notes (by Lucas or others) suggesting changes to the designs, and at least one had the direction "add more greeblies."