Probably alteration of obsolete French desmonterto unseatdes-dis-monterto mountmount1
American Heritage Dictionary of the English Language, 5th Edition
dis- + mount, probably a calque of Old French desmonter.
Dismount Sentence Examples
You should dismount, he said.
He leaned forward as he helped her dismount at the barn, rain rolling off the brim of his hat in a stream.
They must also help the flyers dismount and return to the ground safely.
After the works were silenced, the ships moved in closer, with a view to dismount the Egyptian guns.
The fleet prepared for action, and the bearer of the reply, signed by the president of the council, and offering to dismount three guns in the batteries named, only succeeded in finding the flagship late at night.