late 14c., "clearly revealed," from Old French manifest "evident, palpable," (12c.), or directly from Latin manifestus "plainly apprehensible, clear, apparent, evident;" of offenses, "proved by direct evidence;" of offenders, "caught in the act," probably from manus "hand" (from PIE root *man- (2) "hand") + -festus, which apparently is identical to the second element of infest. De Vaan writes, "If manifestus may be interpreted as 'caught by hand', the meanings seem to point to 'grabbing' or 'attacking' for -festus." But he finds none of the proposed ulterior connections compelling, and concludes that, regarding infestus and manifestus, "maybe the two must be separated."
Manifest destiny, "that which clearly appears destined to come to pass; a future state, condition, or event which can be foreseen with certainty, or is regarded as inevitable" was much used in American politics from about the time of the Mexican War "by those who believed that the United States were destined in time to occupy the entire continent" [Century Dictionary].
Other nations have tried to check ... the fulfillment of our manifest destiny to overspread the Continent allotted by Providence for the free development of our yearly multiplying millions. [John O'Sullivan (1813-1895), "U.S. Magazine & Democratic Review," July 1845]
The phrase apparently is O'Sullivan's coinage; the notion is as old as the republic.