Quizzed about the this week's Clacton by-election on LBC radio, the London Mayor struggled to remember the Tories' "fantastic" candidate, former actor Giles Watling.
Johnson guessed that his name was "Stirling" or "Girling", before pleading with presenter Nick Ferrari: "What’s he called? You tell me.”
After being reminded about his first name, Johnson still had no luck remembering the Tory candidate's surname. "Come on stop sitting there like a great big fat Buddha and tell me the name of this guy," he snapped at Ferrari.
See also - Asa Bennett reports: "Ukip's Douglas Carswell Tells Clacton: ‘Don't Get Angry, Get Change'"
Finally, after being told who Giles Watling was, Johnson concluded: "I knew it. Something to do with ling. Tingaling.”
Johnson may be forgiven for forgetting who the Tories have standing against Carswell, as recent polls suggest the Ukip candidate will enjoy a commanding victory on Thursday, David Cameron's birthday.
Carswell's decision to join Nigel Farage's party and trigger a by-election shocked Westminster and a Survation poll of voters in the Clacton seat predicting a massive 44 point lead for Ukip will add to the prime minister's discomfort.