I doubt very much the suggestion that steward derives from "sty-weard" and originally meant a keeper of pigs. I know that this folk etymology has been suggested but it is not supported by the best evidence, methinks.

Better is the notion that steward derives from "stiweard" in which sti refers to a great hall in a lord's home and weard refers to the person in charge of it. A steward in medieval times was entrusted with the running of the lord's property.

E.g.

"Apart from a very complete staff of servants there were only four of us in the household. These were Miss Witherton, who was at that time four-and-twenty and as pretty--well, as pretty as Mrs. Colmore is now--myself, Frank Colmore, aged thirty, Mrs. Stevens, the housekeeper, a dry, silent woman, and Mr. Richards, a tall military-looking man, who acted as steward to the Bollamore estates. We four always had our meals together, but Sir John had his usually alone in the library."

~from "The Japanned Box" in Six Tales of Mystery by Sir Arthur Conan Doyle.