"We can honour the wishes of the people, or else we can waste more time... an expense of spirit and a waste of shame, more political self-obsession and onanism (masturbation)," he was quoted as saying.
But when he delivered his speech at a taxi manufacturing company in Coventry, west central England, the Conservative leader left out the offensive word.
Pressed about it afterwards, he told reporters: "All I can say is that a stray early draft seems to have somehow found its way into your otherwise peerless copy by a process that I don't pretend to understand. "I will make enquiries," he added.
The words were sent out by the Conservative press office ahead of time to journalists, including AFP.
Johnson is known for his colourful use of language and obscure classical references in Latin and Greek.
He drew criticism earlier this year when he complained on LBC radio about money "spaffed up the wall" on investigating historic child abuse.