what is { mack } ?
Hey, Mack, where is the nearest truck stop?
it is used in 1930s.

Question

what is { mack } ?
Hey, Mack, where is the nearest truck stop? 
  it is used in 1930s.

Answer

Bonus trivia: it comes from the Gaelic "mac", meaning "son".  (ie. Macdonald = son of Donald)  However, "mack" is used among equals.

Answer

Hi Safaa, this was a friendly  American expression used when you didn't know someone's name. It is like  saying Hey buddy or  the Australian expression Hey mate.