noun (sometimes initial capital letters) Informal.
a legislative lobby.
From POTUS To FLOTUS: A Full (White) House Of AcronymsThe faster we move, the more abbreviations and assorted acronyms we use, and when it comes to talking about the folks who reside at 1600 Pennsylvania Avenue—otherwise known as the White House—there are plenty of acronyms to choose from.
Origin of third house
An Americanism dating back to 1850–55
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for third house
US a political lobby for a special interest
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012