These jackets first appeared in the 1930s. Harrington is a nickname. This style of jacket earned it because it was worn by the character Rodney Harrington (Ryan O'Neal) in the 1960s prime time soap opera Peyton Place.
Bonhoeffer uses a similar phrase 'worldly Christianity'. It's J Gresham Machen that I want to line up most closely with. See his Christianity and culture here. Having done commentaries on Proverbs (Heavenly Wisdom) and Song of Songs (Heavenly Love), a matching title for Ecclesiastes would be Heavenly Worldliness. For my stance on worldliness, see 3 posts here.