To convince somebody to reneg on a promise that they made, in similar nature to the way the character Fanny Dashwood convinced her husband John to renege on his deathbed promise to his father that he would take care of his half-sisters and their mother in the Jane Austen novel, Sense and Sensibility.
When Mary convinced her boyfriend, Dave, that he couldn't fulfill his promise to attend his friend Nick's bachelor party, she fanny dashed nick.
by Mr Cnowledge August 17, 2015