Imbecile was an insult used typically to suggest stupidity.
When the Third Doctor saw Harry Sullivan for the first time, through Kaleidoscope's eyes, he said that he was an imbecile. Sarah Jane Smith told him that he was being unfair. (AUDIO: Kaleidoscope)
The Fourth Doctor shouted "Harry Sullivan is an imbecile!", before passing out from the effects of a rockfall on Voga – caused by Harry. (TV: Revenge of the Cybermen) The Twelfth Doctor referred to Z-67, a gas created by Sullivan for use against Zygons, as "the imbecile's gas". (TV: The Zygon Inversion)
Chancellor Padrac told Caleera that Sternbar was an imbecile, making her smile.
The Eleven called Councillor Alekall a "journeyman imbecile with ideas above his station". (AUDIO: Stop the Clock)