"to set or place apart, to detach so as to make alone," by 1786, a back-formation from isolated (q.v.).

The translation of this work is well performed, excepting that fault from which few translations are wholly exempt, and which is daily tending to corrupt our language, the adoption of French expressions. We have here evasion for escape, twice or more times repeated; brigands very frequently; we have the unnecessary and foolish word isolate; and, if we mistake not, paralize, which at least has crept in through a similar channel. Translators cannot be too careful on this point, as it is a temptation to which they are constantly exposed. [The British Critic, April 1799]

As a noun, "something isolated," 1890; from earlier adjectival use (1819), which is from Italian isolato or Medieval Latin insulatus.

"standing detached from others of its kind," 1740, a rendering into English of French isolé "isolated" (17c.), from Italian isolato, from Latin insulatus "made into an island," from insula "island" (see isle (n.)). English at first used the French word (isole, also isole'd, c. 1750), then after isolate (v.) became an English word, isolated became its past participle.

[F]or I think it very natural to suppose, that all the People of this isolated Country, (I ask Pardon for a foreign Word) should have one Language ... ["An Irregular Dissertation Occasioned by the Reading of Father Du Halde's Description of China," 1740]

1899 in reference to U.S. foreign policy, "one who advocates a policy of non-participation in foreign affairs" (earlier in reference to treatment of leprosy), from isolation + -ist. As an adjective from 1920. Isolationism is attested in a general sense by 1902; in a U.S. geopolitical sense by 1919 in reference to opposition to joining the League of Nations.