by 1922 as "an act or the process of melting metal;" by 1956 in reference to the accidental melting of the core of a nuclear reactor, from the verbal phrase (1630s), from melt (v.) + down (adv.). Metaphoric extension "breakdown in self-control" is attested since 1979.

"either of two round projections of a cannon," 1620s, from French trognon "core of fruit, stump, tree trunk," from Old French troignon (14c.), probably from Latin truncus (see trunk (n.1)).

initialism (acronym) of most favored nation, attested from 1942.

"territories subordinate to another nation," 1680s; see dependency.

Paleozoic supercontinent comprising North America and Eurasia, 1931, from German (1928), from Laurentia, geologists' name for the ancient core of North America (see Laurentian) + second element of Eurasia.

"one seeking asylum" in a nation, by 1954, irregularly formed from asylum + -ee.

"the general body of people constituting a nation, state, or community; the nation or state," 1610s, from public (adj.); the meaning "people in general" is from 1660s. In public "in open view, publicly, before the people at large" is attested from c. 1500.

nation in Southeast Asia, from Burmese people's self-designation; see Myanmar.

"of or pertaining to a nation or a country regarded as a whole; established and maintained by the nation; peculiar to the whole people of a country," 1590s, from French national (16c., from Old French nacion), and also from nation + -al (1). Opposed to local or provincial (or in the U.S., state).

Meaning "peculiar or common to the whole people of a country" is by 1620s. From 1802 as "established and maintained by the nation or its laws." As a noun, "citizen of a (particular) nation," from 1887. Related: Nationally. 

National guard is from 1793, originally in reference to an armed force in France identified with the revolution; U.S. use is from 1847, originally a name sometimes given to the organized militia. National anthem is recorded by 1806. 

A King though he's pestered with cares,

    For the most part he's able to ban them;

But one comes in a shape he never can escape—

    The implacable National Anthem!

[W.S. Gilbert, "His Excellency," 1894] 

region in northeastern Germany, late 14c., Prusse (late 13c. as a surname), from Medieval Latin Borussi, Prusi, Latinized forms of the native name of the Lithuanian people who lived in the bend of the Baltic before being conquered 12c. and exterminated by (mostly) German crusaders who replaced them as the inhabitants.

Perhaps from Slavic *Po-Rus "(Land) Near the Rusi" (i.e. Russians; compare Pomerania). The German duchy of Prussia after the 17c. union with the Mark of Brandenberg became the core of the Prussian monarchy and later the chief state in the German Empire. The center of power shifted to Berlin after the union, and the old core of the state came to be known as East Prussia.