WaPo dug up old documents in the National Archives which show that POTUS's great-great-grandfather, Moses J. Robinette, received a pardon by Honest Abe back in the mid-1860s ... this after he got into a fight with a fellow Union Army worker that landed him in jail.

Here's what happened, per ancient history ... Moses was working as a civilian veterinary surgeon for the U.S. Army during the Civil War -- and while he was stationed at the Army of the Potomac's winter camp in Spring 1864, he got into a brawl with another fella.

Long story short ... John J. Alexander -- a brigade wagon master -- apparently overheard Joe's grandad talking s*** about him to the female cook in one of the tents, and he confronted him on the spot -- which led to an argument and eventually a scuffle.

During the fight, Moses was accused of pulling his pocketknife and stabbing John -- as he was left bleeding from several cuts in the aftermath. Soon, authorities arrived to check in.

Moses ended up getting charged with attempted murder -- but during the trial ... that charge ended up being dropped --- although he was convicted on other charges, including assault, "inciting a dangerous quarrel" and "violating good order and military discipline."

A transcript of his trial quotes Moses as saying ... "[W]hatever I have done was done in self defence, that I had no malice towards Mr. Alexander before or since. He grabbed me and possibly might have injured me seriously had I not resorted to the means that I did."

He ended up getting convicted on all counts -- minus the attempted murder -- and he was sentenced to two years incarceration at hard labor. Once they finally figured out where to send him, Moses was shipped to the Dry Tortugas islands near Key West, FL.

Once he was there, other army officers who knew Moses wrote a letter in his defense -- and that worked its way up the channels and eventually landed on Lincoln's desk. He reviewed the case and decided Moses' punishment was way too harsh ... and pardoned him.