The story
From the shape of the Mull of Kintyre peninsula in Scotland, which somewhat resembles a flaccid penis.
Etymology from Wiktionary, CC BY-SA 4.0.
Mull of Kintyre rule
Today it means: According to an urban legend, an unofficial guideline used by publishers and the media in the United Kingdom to decide to what degree of erection an image of a penis can be shown. Full definition →