Definition of "so long" []

• Informal Used to express goodbye. (interjection)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The place in the wardrobe of the gentleman's suit has changed over the past century, and here Sooke employs the expertise of tailor Charlie Allen, Topman's design director Gordon Richardson and Sir Paul Smith to explain why the British male has been so long enamoured with "the whistle"."
• "Reconfiguration of hospital services, so long the NHS's elephant in the room, is back on the agenda."
• "Asked why it took so long to recognize the NTC, Clinton said the U.S. administration analyzed the situation to make sure that the NTC's actions are in accord with its statements."