An elaboration of chinny - the stroking of an invisible beard to denote doubt or disbelief. This is extended to a single long stroke of an imaginary ceremonial Egyptian beard, the stroke coinciding with the final "moon" syllable of "Tutankhamun". Differing levels of disbelief are expressed by the length of beard and duration of the "moon".
"My dad only pretends to be a teacher, he actually smuggles flick-knives across Europe."
"Tutankhamun!" (or - "Tutankha-mooooooooon!" with visual flourish)