To dismount from (a horse). Obsolete.

transitive. To depart from, leave, quit (a place); to dismount from (a horse). Obsolete.

(un-, prefix2 affix 1a.)

To get off, alight from (a horse, etc.).

intransitive. Originally: to dismount from (a horse). Subsequently also: to alight from (a railway carriage, tram, bus, aircraft, etc.).

transitive. To dismount from (a horse or other animal). Also intransitive. Now somewhat rare.