Etymology
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coulda 

in writing, to indicate the common casual pronunciation of could have, by 1909.

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merry widow 

"amorous or designing widow," 1907, from the English title of Franz Lehar's operetta "Die Lustige Witwe" (1905). "The Lusty Widow" would have been more etymological (see lust (n.)), but would have given the wrong impression in English. Meaning "a type of wide-brimmed hat" (popularized in the play) is attested from 1908.

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get over (v.)

1680s, "overcome," from get (v.) + over (adv.). From 1712 as "recover from;" 1813 as "have done with."

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habeas corpus (n.)

writ requiring a person to be brought before a court, mid-15c., Latin, literally "(you should) have the person," in phrase habeas corpus ad subjiciendum "produce or have the person to be subjected to (examination)," opening words of writs in 14c. Anglo-French documents to require a person to be brought before a court or judge, especially to determine if that person is being legally detained. From habeas, second person singular present subjunctive of habere "to have, to hold" (from PIE root *ghabh- "to give or receive") + corpus "person," literally "body" (see corporeal). In reference to more than one person, habeas corpora.

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Kyrie eleison 

early 13c., a Greek liturgical formula adopted untranslated into the Latin mass, literally "lord have mercy" (Psalms cxxii.3, Matthew xv.22, xvii.15, etc.). From kyrie, vocative of kyrios "lord, master" (see church (n.)) + eleeson, aorist imperative of eleo "I have pity on, show mercy to," from eleos "pity, mercy" (see alms). Hence, the corresponding part of a musical setting of the Mass or Anglican Communion.

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red herring (n.)

"smoked herring" early 15c. (they turn red when cured), as opposed to white herring "fresh herring." Supposedly used by fugitives to put bloodhounds off their scent (1680s), hence metaphoric sense (1864) of "something used to divert attention from the basic issue;" earlier it simply meant "a false lead":

Though I have not the honour of being one of those sagacious country gentlemen, who have so long vociferated for the American war, who have so long run on the red-herring scent of American taxation before they found out there was no game on foot; (etc.) [Parliamentary speech dated March 20, 1782, reprinted in "Beauties of the British Senate," London, 1786]
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ignis fatuus (n.)

"will o' the wisp, jack-o-lantern," 1560s, Medieval Latin, literally "foolish fire;" see igneous + fatuous. "It seems to have been formerly a common phenomenon; but is now exceedingly rare" [OED].

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mutatis mutandis 

"with the necessary changes," Latin, literally "things being changed that have to be changed," from the ablative plurals of, respectively, the past participle and gerundive of mutare "to change" (from PIE root *mei- (1) "to change, go, move").

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make out (v.)

c. 1600, "get along, succeed," from make (v.) + out (adv.). Sense of "obtain a clear understanding of" is from 1640s; that of "discern or discover visually" is by 1754; sense of "have sexual relations with" is attested by 1939.

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charley horse (n.)

1887, sporting slang, origin obscure, probably from somebody's long-forgotten lame racehorse. Charley horse seems to have been a name for a horse or a type of horse (perhaps especially a lame one) around that time.

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