in reference to a style of painting aiming to represent overall impressions as they first strike the eye rather than exact details, 1876 (adjective and noun), from French, coined 1874 by French critic Louis Leroy ("école impressionniste") in a disparaging reference to Monet's sunset painting "Impression, Soleil Levant." Later extended to other arts.
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.
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Definitions of impressionistic from WordNet
of or relating to or based on an impression rather than on facts or reasoning;
she had impressionistic memories of her childhood
a surprisingly impressionistic review bearing marks of hasty composition