"loss of memory," 1786 (as a Greek word in English from 1670s), Modern Latin, coined from Greek amnesia "forgetfulness," from a- "not" (see a- (3)) + mnesi- "remembering" (found only in compounds), from stem of mnasthai "to recall, remember," related to mnemnon "mindful," mneme "memory;" from PIE root *men- (1) "to think." The usual word in Greek was amnestia, but this had a specialized sense of "forgetfulness of wrong" (see amnesty).
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.