I was reading with much interest an old article under the title “Jack Tar”. The author says, recognized authorities are strangely unhelpful, regarding the origin of the name. I might say as one who has had much experience in the sailing ships, ‘Jack’ was a general cognomen for all sailors, both navy and merchant marine, as ‘Tommy’ was supplied to the British Soldier; as for instance the old song: “Our Jack’s Come Home From Sea Today.” And the name ‘Tar’ was added to it, because, in the days of the old sailing ships, so much Tar was used all the time, both Coal Tar, and Stockholm Tar, which the sailors called Raw Tar. Many of the ships of those days, were painted below the waterline with Coal Tar and the ropes in the rigging, lanyards, seizing, etc. were treated with Raw Tar and the sailor in those far off days, was working with Tar a lot of the time, hence then name ‘Jack Tar’.

It could be connected with the ships of Tarshish, but sounds far-fetched.