The story
Origin 1927 when Calvin Coolidge, the United States president designated Col. Charles Lindbergh its Goodwill Ambassador during one of his most famous flights. As a compound word representing an honorific title that was merged over time from three lexemes good-will plus ambassador which is still used descriptively by the State of Kentucky to define the Kentucky Colonel as its "ambassador of good-will".
Etymology from Wiktionary, CC BY-SA 4.0.