ACK is the Federal Aviation Administration’s official 3 letter code for Nantucket Memorial Airport. It is taken from letters in the word Nantucket. It is not for Ackerly Field, which is really a mythological name to justify using A-C-K. The US government uses “N” for military airport installations. Thus, Nantucket and Washington’s National Airport use ACK and DCA rather than NAN or NAT.
ACK has become shorthand or slang when referring to Nantucket.