Elaborating on the accepted answer, here is McWhorter in Words on the Move:
There is a big pot and there is a big-ass pot, there is a lame excuse and a lame-ass excuse. An initial temptation is to think this is simply a matter of profanity. However, that implies that leaving aside the fact that one of them is rude, big pot and big-ass pot have the same meaning. They don’t.
You can tell from really trying to imagine just any old adjective, any old time, quietly appended with -ass. Even in the most foulmouthed person you can imagine, notice how hard it is to imagine him saying, I saw a gray-ass squirrel. If you think about it, he’d say that only if he thought of squirrels of some other color as normal, such that the gray squirrel is a surprise. Gray-ass squirrel comes with a backstory:
“Where I come from, squirrels are black, but when I got here, I looked out the window and saw a gray-ass squirrel!” Gray-ass doesn’t mean simply “gray as uttered by a potty-mouth,” but “counterexpectationally gray.”
[...]
But to remark on the pot being “really big” implies, in itself, that there was an expectation that it would not be.
Words change, and ass was assigned a mission. A good guess is that it started with big-ass, because in language as in so much else, things tend to start with the literal and drift into the abstract, and human beings can literally have large behinds: Then a big-ass fellow jumped in and settled it once and for all. However, yes, it would have been fellow rather than guy, dude, or bro, because the counterexpectational ass floated beyond anatomical plausibility as far back as 1919, when someone was documented as getting angry when a “silly ass barber shaved my neck.” All manner of -ass usages pop up well before 1950: an accent criticized as having “lousy broad-ass As,” and familiar-sounding locutions such as green-ass (corporals), poor-ass (southerners), and broke-ass (a waiter). In all these cases, the point is that the quality in question draws attention.
In narrating, we are creating a little movie. We need to focus the camera or the lights on what we deem worthy of note. We don’t want to only show a waiter, but to remark that he was broke, since it isn’t considered the norm for someone who works to be broke. A real camera could zero in on his frayed cuffs or show him hitching a ride home: in a grand old film noir, the character would not be written as actually stating, “I lack funds.” Talking, we might designate the waiter as broke-ass: “He was working and all, but actually he didn’t have any money.”
So we may say that ass is a counterexpectional intensifier; one that goes pretty far back, one that is maybe spreading its wings and moving around the world. I can't attest to BE, but I have heard it used in Indian English (surprising, given its relative chastity).
