Jack denoting any common fellow whose real name was not known has been in use since the 14th century, as has jack meaning a hoist. Jack as in Union Jack meaning a flag dates from the 17th century. Jack or the knave in a deck of playing cards dates from the 18th century. To be on one’s jack, to be alone, is from rhyming slang, Jack Jones/alone, dates from the 1920s, from a Jack Jones mentioned in a popular Gus Elen music hall song that was popular in the early 20th century. See also Tod and Jack tar.