1540s, "to remove or throw down cannons from their mountings," from dis- + mount (v.). Meaning "get off from a horse or other ridden animal" is from 1580s; transitive sense of "throw or bring down from a horse" is from 1610s. Meaning "remove (a gem, picture, etc.) from a frame, setting, or other mount" is by 1879. Related: Dismounted; dismounting.
