late 14c., discussen, "to examine, investigate," from Latin discuss-, past participle stem of discutere "to dash to pieces, agitate, strike or shake apart," in Late Latin and Medieval Latin also "to discuss, examine, investigate," from dis- "apart" (see dis-) + quatere "to shake" (see quash).
Meaning "examine by argument, debate," the usual modern sense, is from mid-15c. (implied in discussing). Sense evolution in Latin appears to have been from "smash apart" to "scatter, disperse," then in post-classical times (via the mental process involved) to "investigate, examine," then to "debate." Related: Discussed.