Etymology
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watergate (n.)
mid-14c., "channel for water;" late 14c., "flood-gate;" from water (n.1) + gate (n.). The name of a building in Washington, D.C., that housed the headquarters of the Democratic Party in the 1972 presidential election, it was burglarized June 17, 1972, which led to the resignation of President Nixon.
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launder (v.)
1660s, "to wash linen," from noun launder "one who washes" (especially linen), mid-15c., a contraction of lavender, from Old French lavandier "washer, launderer" (12c.), from Medieval Latin lavandaria "a washer," which is ultimately from Latin lavare "to wash" (from PIE root *leue- "to wash"). Criminal banking sense first recorded 1961, from notion of making dirty money clean; the word in this sense was brought to widespread use during U.S. Watergate scandal, 1973. Related: Laundered; laundering.
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expletive (n.)

1610s, "a word or phrase serving to fill out a sentence or metrical line," from French explétif (15c.) and directly from Late Latin expletivus "serving to fill out," from explet- past-participle stem of Latin explere "fill out, fill up, glut," from ex- "out" (see ex-) + plere "to fill" (from PIE root *pele- (1) "to fill").

Sense of "an exclamation," especially "a curse word, an oath," first recorded 1815 in Sir Walter Scott, popularized by edited transcripts of Watergate tapes (mid-1970s), in which expletive deleted replaced President Nixon's salty expressions. As an adjective, from 1660s.

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plausible (adj.)

1540s, "acceptable, agreeable; deserving applause or approval" (senses now obsolete), from Latin plausibilis "praiseworthy, pleasing, acceptable," from plaus-, past-participle stem of plaudere "to applaud" (see plaudit). Meaning "having the appearance of truth, apparently right, seemingly worthy of acceptance or approval" is recorded from 1560s; especially "having a specious or superficial appearance of trustworthiness." Related: Plausibly.

The expression plausible deniability emerged during the Watergate scandal (1973) but is said to be from CIA jargon in the 1950s (Allen Dulles sometimes is credited with the first public use); the thing itself is older: "the situation that allows senior officials or powers to deny responsibility for discreditable actions by others in their hierarchy because no one can confirm the deniers knew of the actions."

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