It was invented in the 19th century and is nothing but crap.
You can trace the entire Pre-Trib history from Margaret MacDonald to Edward Irving to John Darby to the 1878-1909 Niagra Conferences, to the influence of C.I. Scofield in the early 1900's (virtually taking over the Niagra Conferences as early as 1895), to the teaching at Moody BI, Denver and Dallas Seminaries.
From my perspective, I wouldn't say it's nothing but crap, however. The idea that Christ could come at any moment, sparked the great evangelistic and missionary movements of the mid- to late 1800s, bringing forth such organizations as the China Inland Mission, Sudan Interior Mission, The Evangelical Alliance Mission, all because of the concept of "imminent return".
Actually, the modern missionary movements were almost all postmil at their inception.