Assassin's Creed: Origins is, as the title so subtly hints, a game about the origins of the Assassins. In case there was any doubt, the new trailer makes it very clear that this is in fact what's going on. Bayek, you see, was the last of the Medjay—an "elite police force" of ancient Egypt, as the Wiki explains—but that just wasn't enough to get the job done.
"There is a new creed now," Bayek intones as the trailer reaches its climax. "From darkness, we have come. And in darkness, we shall stay. We are Medjay no longer. We shall be known as Assassins."
I thought there might be a historical connection at play in the name, since Ubisoft has made a relatively big fuss about the historical underpinnings of the game, but that doesn't appear to be the case. The Online Etymology Dictionary says the term originates from the mid-13th century word hashishiyyin, meaning "hashish-users," although there is a certain murderous element to it too.
"A fanatical Ismaili Muslim sect of the mountains of Lebanon in the time of the Crusades, under leadership of the 'Old Man of the Mountains' (translates Arabic shaik-al-jibal, name applied to Hasan ibu-al-Sabbah), they had a reputation for murdering opposing leaders after intoxicating themselves by eating hashish," the site says. "The plural suffix -in was mistaken in Europe for part of the word (compare Bedouin). Middle English had the word as hassais (mid-14c.), from Old French hassasis, assasis, which is from the Arabic word."
It goes without saying that you probably won't see anything like that in Assassin's Creed: Origins. Ubisoft can't be having it slapped with an AO rating, after all.