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c.1300, "meeting of adversaries, confrontation," from Old French encontre "meeting; fight; opportunity," noun use of preposition/adverb encontre "against, counter to" from Late Latin incontra "in front of," from Latin in- "in" (see in- (2)) + contra "against" (see contra). Modern use of the word in psychology is from 1967, from the work of U.S. psychologist Carl Rogers (1902-1987). Encounter group attested from 1967.
c.1300, "to meet as an adversary," from Old French encontrer "confront," from encontre (see encounter (n.). Weakened sense of "casually meet" first recorded in English early 16c. Related: Encountered; encountering.