The story
The term refers to the location of Barking station on the District line of the London Underground, which is three stops west of Dagenham Heathway tube station, in allusion to the phrase barking mad.
Etymology from Wiktionary, CC BY-SA 4.0.
three stops short of Dagenham
three stops short of Dag·en·hamToday it means: Crazy; mad. Full definition →