I found this in dictionary.com
First went to prop which gave this:
prop2 /prɒp/ Show Spelled Pronunciation [prop]
property (def. 8).
which then linked 'property' to:
8. Also called prop.
a usually movable item, other than costumes or scenery, used on the set of a theater production, motion picture, etc.; any object handled or used by an actor in a performance.
Interesting question tho, I wonder if there is someone out there with a more detailed answer