The story
Humorous euphemism associating the position of the hands on a toilet seat while vomiting with holding the steering wheel of a bus.
Etymology from Wiktionary, CC BY-SA 4.0.
drive the porcelain bus
drive the por·ce·lain busToday it means: Synonym of worship the porcelain god (“to vomit into a toilet bowl, especially while drunk or hungover”). Full definition →