Etymology
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clasp-knife (n.)

"knife with one or more blades which fold into the handle," 1743, from clasp (n.) + knife (n.). The thing itself was known to the Etruscans and Romans; it became popular again 17c. 

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paradoxical (adj.)

"of the nature of a paradox," 1580s, from paradox + -ical. Meaning "inconsistent with itself" is by 1630s. Competing forms were paradoxal (1560s), paradoxial (1620s), but these survive in niches, if at all. Related: Paradoxically.

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authoritarian (adj.)

"favoring imposed order over freedom," 1862, from authority + -an. Compare authoritative, which originally had this meaning to itself. The noun in the sense of "one advocating or practicing the principle of authority over individual freedom" is from 1859.

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high-five (n.)
1980, originally U.S. basketball slang, 1981 as a verb, though the greeting itself seems to be older (Dick Shawn in "The Producers," 1968). From high (adj.) + five, in reference to the five fingers of the hand.
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Bar Mitzvah 
1842, in Judaism, "male person who has completed his 13th year" and thus reached the age of religious responsibility; Hebrew, literally "son of command." As a name for the ceremony itself, by 1917.
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Lancelot 
masc. proper name, Old French, a double-diminutive of Frankish Lanzo, itself a shortened pet-name (hypocoristic) of one of the many Germanic names in Land- (compare Old English Landbeorht "land-bright;" see Lambert).
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good den 
salutation, Elizabethan corruption of good e'en, itself a contraction of good even "good evening," which is attested from c. 1500, first as gud devon, showing a tendency toward misdivision. See good (adj.) + even (n.).
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introvert (n.)
1878, in zoology, "part or organ which is turned in upon itself," from introvert (v.). The psychological sense "introverted person" (opposed to extrovert) is 1917, from German, introduced there by C.G. Jung (1875-1961).
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autoclave (n.)

"stewing apparatus the lid of which is kept closed and tight by the steam itself," 1847, from French (1821), literally "self-locking," from auto- "self" (see auto-) + clave, from Latin clavis "key" (from PIE root *klau- "hook").

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mast-head (n.)

also masthead, 1748, "top of a ship's mast" (the place for the display of flags), hence, from 1838, "top of a newspaper" (where its name, etc. appears; the nameplate itself is commonly the flag), from mast (n.1) + head (n.).

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