1An officer in charge of the horses belonging to a sovereign, etc.; (also) an official of the British royal household who attends the monarch on state occasions and who has titular responsibility for matters of royal transport, etc. (also "Master of the King's (also Queen's) Horse(s)", formerly sometimes "†master of horses").
A commander of the cavalry appointed by a dictator of the Roman Republic.
3 humorous A head groom or stableman.
Late Middle English; earliest use found in Rolls of Parliament. Originally with unchanged plural of horse.