What does -t-s-s mean?

Answer 1

This is an Old English word of Germanic origin (from at least the 1580's), and not an acronym. It means "to defecate" or "to deceive".

The following answer is complete nonsense:

It's an acronym for when they would store cargo in the back of the ship and the steps of displeasure about it spelled out s-h-i-t.

Further more, I'll add what was left unsaid with some insight on the history... In the 17th Century, the Pilgrims emigrated to America from Europe (in these times Europeans were big on harvests/crops). But they weren't inept with trafficking manure over water-boundaries to spread out the harvesting crops.

[Continue reading, but keep in mind that the word "shit" was already hundreds of years old at this point]

So when they built Ferries to move the manure they at first stowed it in to the bottom of the boat. When it would get stormy and the water would wake, the water would spill over the ferries sides and down in to the manure at the bottom of the ferry therefore mixing an awful stench of Water and Manure which would, You guessed it, Rise up throughout the entire ferries hull and deck and where all of the other patrons would be residing. Hence, s-h-i-t.

After they discovered that stowing manure in the bottom of the Ferry was a horrid idea they stuck signs up. Saying something like the following:

If transporting Manure STORE HIGH IN TRANSIT. Thus we get s-h-i-t. and either coincidental or arbitrarily remembered it's just stuck with society to mend to relate that "I've gotta go take a Manure/s-h-i-t."