"a bursting inward, a sudden collapse," 1829, modeled on explosion, with assimilated form of in- "into, in, on, upon" (from PIE root *en "in").
And to show how entire the neglect and confusion have been, they speak in the same breath of all these explosions, and of the explosion of a mixture of hydrogen and oxygen, the result of which, instead of being a gas or an enlargement of bulk, a positive quantity, is a negative one. It is a vacuum, in a popular sense, because the produce is water. The result is an implosion (to coin a word), not an explosion .... ["Gas-light," Westminster Review, October 1829]
In early use often in reference to effect of deep sea pressures, or in phonetics. Figurative sense is by 1960.
also mid-term, "in the middle of a term" in any sense, from mid (adj.) + term (n.). By 1879 in reference to gestation; 1888 of college semesters (midterm examination is by 1900; student slang shortening midterms for these is by 1903). By 1891 in reference to U.S. congressional elections held in the middle of a four-year presidential term.
1690s, in reference to a specific religious movement, Pietism, from German Pietismus, originally applied in derision to the movement to revive personal piety in the Lutheran Church, begun in Frankfurt c. 1670 by Philipp Jakob Spener (1635-1705). See piety + -ism. With lower-case p- and in reference generally to devotion, godliness of life (as distinguished from mere intellectual orthodoxy), by 1829.
