Definition of dum-dum in English:

dum-dum

noun

informal
  • A stupid person.

    ‘the dum-dum at the back of the class’
    • ‘Because we are free, people can choose to be dum-dums.’
    • ‘Of course I care for her dum-dum, she's my responsibility!’
    • ‘Take advantage of him while he's here you dum-dum!’
    • ‘‘How can I forget my own daughter's birthday you dum-dum!’’
    idiot, ass, halfwit, nincompoop, blockhead, buffoon, dunce, dolt, ignoramus, cretin, imbecile, dullard, moron, simpleton, clod

Origin

1970s (originally US): reduplication of dumb.