zachttp://www.oed.com/dictionary/zac_n%3Ftab%3Dmeaning_and_usezac, n.
First published 1986; not fully revised

zacnoun

Australian slang.
  1. 1898–
    A sixpence.
    1. 1898
      6d., a ‘zack’.
      Bulletin (Sydney) 1 October 14
    2. 1924
      Zack, sixpence.
      Truth (Sydney) 27 April 6
    3. 1941
      ‘Only one, then,’ I said. ‘We'll only have one more, so we'll make it a sixpence in.’ ‘Don't worry about the zac,’ said Tom.
      Coast to Coast 213
    4. 1952
      Arthur spent the break losing bobs and tanners while the tall Australian reckoned his profits in dieners and zaks.
      M. Tripp, Faith is Windsock ii. 29
    5. 1962
      And all you get for it is a zack.
      John o' London's 8 March 229/2
    6. 1966
      The zack, now the 5c piece, is in such demand that very often its scarcity makes one wonder if it is gradually going out of circulation.
      Telegraph (Brisbane) 13 October 13/2
    7. 1966
      Two taps with an improvised hammer and anvil, a two bob and a zac.
      P. Mathers, Trap ii. 31
    8. 1977
      When it comes to unique competitions the people of outback Winton reckon they're the full quid—and you can bet your last zac or traybit on it.
      Sunday Sun (Brisbane) 1 May 16

zac, n. was first published in 1986; not fully revised.

zac, n. was last modified in June 2024.