The story
An allusion to Ephesians 4:26–27, viz. “…let not the sun go down upon your wrath: // Neither give place to the devil.”
Etymology from Wiktionary, CC BY-SA 4.0.
let not the sun go down upon one's wrath, neither give place to the devil
Today it means: Seek to dispel ill-will before a day’s end, and not to act upon desires for vengeance. Full definition →