Etymology
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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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ab urbe condita 
with year-dates, an occasional Roman method of identifying a given year by reference to the time passed since founding of the city, which in 1c. B.C.E. was calculated to have taken place in what we would call 753 B.C.E. Literally "from the city founded;" the elements are ab "from" (see ab-) + ablative of urbs "city" (see urban) + fem. past participle of condere "put together, store," from assimilated form of com- "together" (see com-) + -dere "put," from PIE root *dhe- "to put, place."
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non compos mentis 

c. 1600, in law, "not capable, mentally, of managing one's own affairs," Latin, "not master of one's mind," from non "not" + compos "having power" (from com- "together" + potis "powerful") + mentis "of the mind," genitive of mens "mind."

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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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commedia dell'arte (n.)

"improvised popular comedy involving stock characters," 1823, Italian, literally "comedy of art;" see comedy + art (n.).

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common sense (n.)

late 14c., originally an internal mental power supposed to unite (reduce to a common perception) the impressions conveyed by the five physical senses (Latin sensus communis, Greek koine aisthesis). Thus "ordinary understanding, without which one is foolish or insane" (1530s); the meaning "good sense" is from 1726. Also, as an adjective, common-sense "characterized by common sense" (1854).

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black comedy (n.)

1961, "comedy that deals in themes and subjects usually regarded as serious or taboo," from black (adj.), in a figurative sense of "morbid," + comedy. Compare French pièce noire, also comédie noire "macabre or farcical rendering of a violent or tragic theme" (1958, perhaps the inspiration for the English term) and 19th-century gallows-humor. In a racial sense, from 1921.

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hors de combat (adv.)
1757, French, literally "out of combat." Hors (prep.) "out, beyond," is from Latin foris (adv.) "outside," literally "out of doors" (see foreign). De is from Latin de "of." For combat see combat (n.). A similar expression from French is hors concours "out of competition" (1884), of a work of art in an exhibition.
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common law (n.)

mid-14c., "the customary and unwritten laws of England as embodied in commentaries and old cases" (see common (adj.)), as opposed to statute law. Phrase common-law marriage is attested from 1909.

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