Discussion Forum—A Way with Words, a fun radio show and podcast about language

Here's some gleanings from the internet- which may or may not be accurate, though the quote from Pepys seems to be relevant. Apparently not all captains were Gentlemen, and the crews were better for it...

"Jack was a common name applied to any working man.

Sailors were known as Jack Tars because of the splashes of tar on their clothes. They also applied tar to their thick overcoat to help make it waterproof.

Long hair could get caught in the ship's equipment when working onboard so to stop this from happening sailors would tar their pigtails. Officers were excused from this "hair care". Sailors were still allowed long hair (tied into pigtails and tarred) up until the early-20th century.

Another tidbit:
Because sailors often worked with dirty/tarred rope, their hands would get filthy. Queen Victoria didn't like to see sailors filthy palms when they saluted so the Navy changed it's salute so as to hide the palms. This why the Navy's hand salute is different from the Army's and Air Force's.

Samual Pepys used the word tarpaulin to refer to a sailor - see diary entry of June 29th, 1667."

"Then we to talk of the loss of all affection and obedience, now in the seamen, so that all power is lost. He told us that he do concur in thinking that want of money do do the most of it, but that that is not all, but the having of gentlemen Captains, who discourage all Tarpaulins, and have given out that they would in a little time bring it to that pass that a Tarpaulin should not dare to aspire to more than to be a Boatswain or a gunner. That this makes the Sea Captains to lose their own good affections to the service, and to instil it into the seamen also, and that the seamen do see it themselves and resent it."  Pepys Diary