1770, "a bodily purging" (especially of the bowels), from Latinized form of Greek katharsis "purging, cleansing," from stem of kathairein "to purify, purge," from katharos "pure, clear of dirt, clean, spotless; open, free; clear of shame or guilt; purified" (with most of the extended senses now found in Modern English clear, clean, pure), which is of unknown origin.
Originally medical in English; of emotions, "a purging through vicarious experience," from 1872; psychotherapy sense first recorded 1909, in Brill's translation of Freud's "Selected Papers on Hysteria."
The German abreagiren has no exact English equivalent. It will therefore be rendered throughout the text by "ab-react," the literal meaning is to react away from or to react off. It has different shades of meaning, from defense reaction to emotional catharsis, which can be discerned from the context. [footnote, pp. 5-6]