Etymology
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think tank (n.)
also think-tank, 1959 as "research institute" (first reference is to Center for Behavioral Sciences, Palo Alto, Calif.); it had been colloquial for "the brain" since 1905. See think + tank (n.).
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cogito ergo sum 

Latin phrase, literally "I think, therefore I am;" the starting point of Cartesian philosophy (see Cartesian), from cogito, first person singular present indicative active of cogitare "to think" (see cogitation) + ergo "therefore" (see ergo) + sum, first person singular present indicative of esse "to be" (from PIE root *es- "to be").

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mens rea 

"state of mind accompanying an act which condemns the perpetrator to criminal punishment," Latin, literally "guilty mind;" from mens "mind," from PIE root *men- (1) "to think."

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memento mori (n.)

"a reminder of death," 1590s, a decorative object, usually an ornament for the person, containing emblems of death or reminders of the fleetingness of life, common in 16c., a Latin phrase, literally "remember to die," that is, "remember that you must die." From second person singular imperative of meminisse "to remember, recollect, think of, bear in mind" (a reduplicated form, related to mens "mind," from PIE root *men- (1) "to think") + mori "to die" (from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death).

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Ahura Mazda 
from Avestan ahura- "spirit, lord," from Indo-Iranian *asuras, from suffixed form of PIE root *ansu- "spirit" (see Aesir) + Avestan mazda- "wise," from PIE *mens-dhe- "to set the mind" (from root *men- (1) "to think" + root *dhe- "to set, put").
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compos mentis (adj.)

Latin, literally "in command of one's mind," from compos "having the mastery of," from com "with, together" (see com-) + stem of potis "powerful, master" (from PIE root *poti- "powerful; lord"), + mentis, genitive of mens "mind" (from PIE root *men- (1) "to think").

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ish kabibble 

slang phrase meaning, more or less, "I don't care, I don't worry," 1913, of unknown origin, but perhaps derived from Yiddish nisht gefidlt. Said to have been popularized by comedienne Fanny Brice (1891-1951), but earliest references do not mention her.

Chicken pox doesn't poison the wellsprings of one's existence like 'Ish kabibble,' and 'I should worry.!' Do you think it's any fun to bring up children to speak decent English, and then have their conversation strewed with phrases like that and with ain'ts? Do you think I like to hear Robert talking about his little friends as 'de guys' and 'de ginks?' [Mary Heaton Vorse, "Their Little Friends," in Woman's Home Companion, February 1916]
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fast and loose 
described as "a cheating game played with a stick and a belt or string, so arranged that a spectator would think he could make the latter fast by placing a stick through its intricate folds, whereas the operator could detach it at once." [James O. Halliwell, "Dictionary of Archaic and Provincial Words," 1847]. The figurative sense (1550s) is recorded earlier than the literal (1570s).
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old-world (adj.)

1712, "belonging to a prehistoric age," see old + world. Meaning "of or pertaining to Eurasia and Africa," as opposed to the Americas, is by 1877. The noun phrase Old World in this sense is by 1590s. The division of the earth into Old World and New World among Europeans dates to 1503 and Italian explorer Amerigo Vespucci's use of Latin Mundus Novus for the lands of the western hemisphere found by Columbus and others, indicating they were not part of Asia.

The Known World is usually divided into four Parts, Europe, Asia, Africk and America. But it is a most unequal Division, and I think it more rational to divide it thus. Viz. the Known World, first into two Parts, the Old and the New World; then the Old World into three, Europe, Asia, and Africa; and the New into two, the Northern and Southern America. [Guy Miege, "A New Cosmography, or Survey of the Whole World," London, 1682]
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