"I'm late now, Fatty. C'mon. One side," Zooey said. A Philadelphia highboy had been moved out into the hall, and, together with Mrs. Glass's person, it blocked Zooey's passage.
(idiomatic) A place where things are stored or reserved, as in:
He put next year's reserved seed on one side.
He put some money to one side.
Etymology
Probably short for "move to one side", that is to one side of a passageway.