Etymology
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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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in memoriam 

Latin, literally "in memory of," from accusative of memoria "memory" (from PIE root *(s)mer- (1) "to remember"). The phrase was much-used in Latin writing; Tennyson's poem of that name (published in 1850) seems to have introduced the phrase to English.

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al Qaida 

also Al-Qaeda; name of a loosely structured jihadist movement founded c. 1989 by Osama bin Laden; from Arabic, literally "the base." A common Arabic term among Muslim radicals from the wider Islamic world who came to Afghanistan in 1980s and fought alongside local rebels against the Soviets, and who regarded themselves and their struggle not merely in Afghan terms but as the "base" or foundation of a wider jihad and revival in Islam. Used by Bin Laden's mentor, Abdallah Azzam, who referred to the "vanguard" which "constitutes the strong foundation [al-qaida al-sulbah] for the expected society." In U.S., the term first turns up in a CIA report in 1996.

Every Muslim, from the moment they realise the distinction in their hearts, hates Americans, hates Jews, and hates Christians. This is a part of our belief and our religion. For as long as I can remember, I have felt tormented and at war, and have felt hatred and animosity for Americans. [Osama bin Laden, interview aired on Al-Jazeera, December 1998]
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so long (interj.)

parting salutation, 1860, of unknown origin, perhaps from a German idiom (compare German parting salutation adieu so lange, the full sense of which probably is something like "farewell, whilst (we're apart)"); or perhaps from Hebrew shalom (via Yiddish sholom). Some have noted a similarity to Scandinavian leave-taking phrases, such as Norwegian Adjø så lenge, Farvel så lenge, Mor'n så lenge, literally "bye so long, farewell so long, morning so long;" and Swedish Hej så länge "good-bye for now," with så länge "for now" attested since 1850 according to Swedish sources. Most etymology sources seem to lean toward the German origin. So long (adv.) "for such a long time" is from late Old English.

Earlier guesses that it was a sailors' corruption of a South Pacific form of Arabic salaam are not now regarded as convincing. "Dictionary of American Slang" also adds to the list of candidates Irish slán "safe," said to be used as a salutation in parting. The phrase seems to have turned up simultaneously in America, Britain, and perhaps Canada, originally among lower classes. First attested use is in title and text of the last poem in Whitman's "Leaves of Grass" in the 1860 edition.

An unknown sphere, more real than I dream'd, more direct, darts awakening rays about me — So long!
Remember my words — I may again return,
I love you — I depart from materials;
I am as one disembodied, triumphant, dead.

Whitman's friend and fan William Sloane Kennedy wrote in 1923:

The salutation of parting — 'So long!' — was, I believe, until recent years, unintelligible to the majority of persons in America, especially in the interior, and to members of the middle and professional classes. I had never heard of it until I read it in Leaves of Grass, but since then have quite often heard it used by the laboring class and other classes in New England cities. Walt wrote to me, defining 'so long' thus: "A salutation of departure, greatly used among sailors, sports, & prostitutes — the sense of it is 'Till we meet again,' — conveying an inference that somehow they will doubtless so meet, sooner or later." ... It is evidently about equivalent to our 'See you later.' The phrase is reported as used by farm laborers near Banff, Scotland. In Canada it is frequently heard; 'and its use is not entirely confined to the vulgar.' It is in common use among the working classes of Liverpool and among sailors at Newcastle-upon-Tyne, and in Dorsetshire. ... The London Globe suggests that the expression is derived from the Norwegian 'Saa laenge,' a common form of 'farewell,' au revoir. If so, the phrase was picked up from the Norwegians in America, where 'So long' first was heard. The expression is now (1923) often used by the literary and artistic classes.
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