Etymology
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quid pro quo 

"one thing in place of another," 1560s, from Latin, literally "something for something, one thing for another," from nominative (quid) and ablative (quo) neuter singulars of relative pronoun qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns) + pro "for" (see pro-).

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basket case (n.)
1919, American English, originally a reference to rumors of quadriplegics as a result of catastrophic wounds suffered in World War I (the U.S. military authorities vehemently denied there were any such in its hospitals), from basket (n.) + case (n.2). Probably literal, i.e., stuck in a basket, but basket had colloquial connotations of poverty (begging) and helplessness long before this. Figurative sense of "person emotionally unable to cope" is from 1921.
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quo warranto 

mid-15c. (late 13c. in Anglo-French), "royal writ to determine by what warrant a person holds an office or franchise," a Medieval Latin legal phrase, literally "by what warrant," from quo "from, with, or by whom or what?," ablative of the interrogative pronoun quis "who?" (from PIE root *kwo-, stem of relative and interrogative pronouns). Also see warrant (n.).

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cui bono 

a Latin phrase from Cicero. It means "to whom for a benefit," or "who profits by it?" not "to what good purpose? for what use or end?" as is sometimes said. From cui "to? for whom?," an old form preserved here in the dative form of the interrogative pronoun quis "who?" (from PIE root *kwo-, stem of relative and interrogative pronouns) + bono "good" (see bene-).

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sine qua non 
"an indispensable condition," Latin, literally "without which not," from sine "without" (see sans) + qua ablative fem. singular of qui "which" (from PIE root *kwo-, stem of relative and interrogative pronouns) + non "not" (see non-). Feminine to agree with implied causa. The Latin phrase is common in Scholastic use. Sometimes a masculine form, sine quo non, is used when a person is intended. Proper plural is sine quibus non.
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comme il faut 

"according to etiquette," 1756, French, literally "as it should be." From comme "as, like, how," from Old French com, from Vulgar Latin *quomo, from Latin quomodo "how? in what way?," pronominal adverb of manner, related to quam "how much?," qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns). With il, from Latin ille "this" (see le) + faut, third person singular present indicative active of falloir "be necessary," literally "be wanting or lacking" (see fail (v.)).

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