Definition of ack-ack in English:

ack-ack

noun

Military
informal
  • [mass noun], [usually as modifier] Anti-aircraft gunfire:

    ‘a quick burst of ack-ack fire’
    • ‘Keating and the officers and men went to sleep under clean skies, a sleep disturbed by the angry bark of ack-ack batteries and the drone of German planes looking for the beaches.’
    • ‘Heavy and light Japanese ack-ack trailed the runaway plane.’
    • ‘But the war grew wings and swept on and except for one Ju 88 who buzzed the field without biting yet drew 5000 rounds from the eager ack-ack boys.’
    • ‘Gardner was hit by our own ack-ack and bailed out.’
    • ‘The flight across the Channel was uneventful, but as soon as the formation crossed into German territory, ack-ack started opening up.’

Origin

Signallers' name for the letters AA; ack for A was replaced in military use by able in 1942.

Pronunciation:

ack-ack

/akˈak/