nostalgiayoudaoicibaDictYouDict[nostalgia 词源字典]
nostalgia: [18] Etymologically, nostalgia is pain connected with returning home – in other words, homesickness. It is a modern coinage, based ultimately on Greek nostos ‘homecoming’ and algos ‘pain, grief’ (as in analgesic [19] and neuralgia (see NEURAL)). At first it was used as the name of what was regarded virtually as a form of mental illness (the earliest known record of it is in the journal kept by the botanist and explorer Joseph Banks on Captain Cook’s round-the-world voyage, in which he noted (1770) that most of the ship’s company were ‘now pretty far gone with the longing for home which the Physicians have gone so far as to esteem a disease under the name of Nostalgia’).

The milder present-day connotations of wistful longing for a past time emerged in the early 20th century.
[nostalgia etymology, nostalgia origin, 英语词源]
nostalgia (n.)youdaoicibaDictYouDict
1770, "severe homesickness considered as a disease," Modern Latin, coined 1668 in a dissertation on the topic at the University of Basel by scholar Johannes Hofer (1669-1752) as a rendering of German heimweh (for which see home + woe). From Greek algos "pain, grief, distress" (see -algia) + nostos "homecoming," from PIE *nes- "to return safely home" (cognate with Old Norse nest "food for a journey," Sanskrit nasate "approaches, joins," German genesen "to recover," Gothic ganisan "to heal," Old English genesen "to recover"). French nostalgie is in French army medical manuals by 1754.

Originally in reference to the Swiss and said to be peculiar to them and often fatal, whether by its own action or in combination with wounds or disease. By 1830s the word was used of any intense homesickness: that of sailors, convicts, African slaves. "The bagpipes produced the same effects sometimes in the Scotch regiments while serving abroad" [Penny Magazine," Nov. 14, 1840]. It is listed among the "endemic diseases" in the "Cyclopaedia of Practical Medicine" [London, 1833, edited by three M.D.s], which defines it as "The concourse of depressing symptoms which sometimes arise in persons who are absent from their native country, when they are seized with a longing desire of returning to their home and friends and the scenes their youth ...." It was a military medical diagnosis principally, and was considered a serious medical problem by the North in the American Civil War:
In the first two years of the war, there were reported 2588 cases of nostalgia, and 13 deaths from this cause. These numbers scarcely express the real extent to which nostalgia influenced the sickness and mortality of the army. To the depressing influence of home-sickness must be attributed the fatal result in many cases which might otherwise have terminated favorably. ["Sanitary Memoirs of the War," U.S. Sanitary Commission, N.Y.: 1867]
Transferred sense (the main modern one) of "wistful yearning for the past" first recorded 1920, perhaps from such use of nostalgie in French literature. The longing for a distant place also necessarily involves a separation in time.