I think it came from a story or film or somesuch where the arse being rescued from the sea reaches the deck of the rescue ship and says to the Matelot "I'm alright Jack, pull up the ladder" not worrying about the others left in the water. Hence Jack Man.
In England in the 1950s, the phrase âIâm alright , Jackâ referred sarcastically to the attitude of people who believed all was well in the world as long as they got theirs. âAs long as Iâve got mine, everything is fine.â
'Jack' is a catch-all English name for almost anyone - Jack of all trades, Jack Tar etc. It comes from the French 'Jaques' a traditional name for a peasant centuries ago. It spead to things other than people.
It's the role of the new boy in a sabre troop of an armoured car regiment. When the vehicle gets a flat tyre, the new boy has to climb under the wheel station and push up until the rest of the crew can undo the bolts and replace the wheel.