c. 1600, "an adherent of an emperor or the imperial cause," such as the emperor of Germany (in the Thirty Years War), France, China, etc., probably modeled on French impérialiste (early 16c.); from imperial + -ist. The shift in meaning to "advocate of imperialism" (1893) came via the British Empire, which involved a worldwide colonial system. See imperialism. As a term of abuse in communist circles, attested by 1918. As an adjective by 1816.
-ic
Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).
In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.