dum-dum

   ‘You take it to avoid getting preg nant, dum-dum,’ says a young woman to a boy in Slouching Towards Kalamazoo, by Peter de Vries. The narrator adds the comment: ‘Another new expression not entirely unknown to us in the sticks.’ A ‘dum-dum’ is actually a kind of bullet that expands when it hits an object, but the vocative is clearly inspired by ‘dummy’, ‘Dummkopf’, etc. and implies stupidity.