Well, cocksucker, the fucking dialog is comparable to what the fucking dialog would have been 150 years ago.
You see, if they used a word that packed the same punch back then as something like cocksucker does now, you as the viewer would wonder, "What's so bad about calling someone a sidebarrel pulling gekko? What does that even mean?"
Ya dig?
So, no, they didn't run around saying cocksucker every other word in 1870, but the equivalent modern language was used for your benefit. So it would have the same impact on you as it did on people back in the day.
But the language - not the swearing - is the most awe inspiring thing about that show. Okay, the language and the brutal, unpredictable violence. And the mud. And everything else about it.
The old west was never done better. And yes, South Dakota was still "the west" in those days.