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This is how you could to that. Avoid using String and putting the string constant in program memory.

// A buffer for the message
const size_t MSG_MAX = 16;
char msg[MSG_MAX];

void setup()
{
  Serial.begin(9600);
  while (!Serial);
}

void loop()
{
  static int len = 0;
  // Check if it is time to create the initial message
  if (len == 0) {
    strcpy_P(msg, (const char*) F("hello"));
    strcat_P(msg, (const char*) F(" jack"));
    len = strlen(msg) - 1;
  }
  // Otherwise shorten the message
  else {
    msg[len--] = 0;
  }
  // And print current message
  Serial.println(msg);
  delay(2000);
}