The deck of a ship at night can be a pretty inhospitable place, especially in cold or inclement weather. As the illustration above, entitled Nightwatch, shows admirably. There's the midshipmen in the foreground, poorly outfitted to the point that he lacks a boatcloak (until the latter part of the 19th century, each individual sailor including officers was responsible for his own wardrobe). The officer of the watch and the sailing Master, with his top hat and speaking trumpet, look a bit more appropriately dressed. Most discomfited of all, though, is the steward, racing toward the Captain's great cabin with a bottle in his hand and only his shirt to keep him warm.
All this is bad enough but add to it the dislike and distrust of your shipmates and you could really be in a terrible position. Terrible enough to end up dead.
In seaman's terms a Jonah is someone who brings bad luck to the ship. Usually he is a man who is perceived as personally unlucky. Maybe a midshipman who has not been able to pass the Lieutenant's exam and now, late in his twenties he is still in a position generally reserved for boys in their teens. Maybe he's come from more than one ship that wrecked or sank and he's become a walking ill omen. Maybe he's just that jerk nobody wants to sit next to at dinner. The politics aboard ship could be surprisingly similar to that of our modern cubicle farms. What ever the reason, the Jonah takes the ship's wind, lets the enemy find her and allows prizes to escape unmolested. As one disaster piles up on another, superstitious seamen start to plot a way to turn the ship's luck.
The term Jonah comes, of course, from the Old Testament. Jonah was a prophet on a sea voyage who managed to call up a storm by offending God. The seamen aboard, in the hope of saving their own skins, tossed Jonah overboard where he was famously swallowed by a whale. For those of you of a certain age, that Burning Sensations song is going through your head right now. Suddenly, you feel like Jonah in the belly of the whale.
Just like in the Bible, the cure for the Jonah's ill luck was to cast him overboard to drown. This frequently occurred at the head and late at night when help would not be immediately available if the victim put up a fuss. Men who thought themselves targets would try to avoid going to the bathroom at all costs, which must have been decidedly uncomfortable. The actual act of tossing the bad luck overboard was called a Jonah's lift. This referred not just to lifting the man in question over the rail but lifting the cloak of misfortune from the ship as well.
Life at sea wasn't just hard it was harsh. Keep your noses clean, Brethren. Its worth it.