Learn English – Why do we “beat seven bells out of” someone

OED

The Oxford English Dictionary defines far out as:

Of jazz: of the latest or most progressive kind. More generally, avant-garde, far-fetched; excellent, splendid. orig. U.S.

Their first quotation is an 8th November 1954 Time Magazine article entitled "Far-Out Words for Cats":

Jazz lingo becomes obsolescent almost as fast as it reaches the public ear.‥ A daring performance was ‘hot’, then ‘cool’, and now is ‘far out’.

1940s

We can see this term beginning to emerge in Simon Michael Bessie's 1938 book Jazz Journalism: The Story of the Tabloid Newspapers:

The third class, divertingly far out on the lunatic fringe, embraces such journalistic peep-shows as Bernarr Macfadden's New York Evening Graphic.

Whilst the book is about tabloid papers and not jazz, it shows "far out" being applied to something novel and possibly avant-garde.

Here's Esquire's 1946 Jazz Book applying it to jazz of the most progressive kind:

Much of its drive came from two other jazzmen on this list: drummer Davey Tough and bassist Chubby Jackson. Both are incomparable musicians; both have contributed mightily to that spark and zest which are so characteristic of Woody's band and which take it far out of the realm of the typical swing band.

Here's another snippet, from a 1947 The Jazz Record:

George [Lugg] sat staring miserably at the floor, and when the trombone solos came along he made impatient gestures and said: "That wasn't what I wanted to do at all." But in some of those ensemble passages, banging away behind Goodwin's trumpet and Scott's clarinet, with a good rhythm section pushing, he began to grin and nod his head. "That's it!" he'd say. 'That's good." He was so impersonal, so far out of himself, a musician listening to music and judging it by what he heard.

And from the next page:

Shuffle up that "Chain Gang" thing. Hear this cat blowing blue in the lost reaches of the world's last swamp. Go with this Yank into the eternal night, so far out of the world that nothing remains but the half-remembered crying of a horn. Born of slavery's misery comes jazz, bringing the gap from cat to cat with a blue bridge.

These may not be the exact use of the phrase, but give an indication of how it came around.

1954

It appears to have used as a set term in a publication of the South Dakota State University sometime in 1954:

... Ella Fitzgerald and hundreds of other "cats." To put expression for our word "darin;[?] into jazz terms, the jazz today is "far-out".

Billboard of 24 April 1954 reviews Les Elgart Orchestra's song "East is East":

A pretty instrumental with an exotic Oriental flavor. The virtuosity of Elgart's intrumentalists makes for easy listening, but there is a solid beat here for dancing. A little on the modern side, but not too far out.

Billboard of 26 June 1954 reviews Serena Shaw's "St. Louis Blues":

Here is the perennial as you might expect Yma Sumac to sing it. Miss Shaw, aided by exotic sounds from drums, organ and echo chamber, soars up and zooms down a two-octave range at a dizzying pace, working up to a frantic pitch at the end. It's pretty far out, but deejays might find it unusual programming.

The same November 8, 1954 Time Magazine cited by the OED had another article that used:

Says Jazz Promoter Norman Granz, who does not always understand [David] Brubeck's "far-out" music: "He's way out on Cloud 7."

OED's definition #5 for cracked...

Of the brain, mind, etc.: Unsound, impaired, somewhat deranged.
Of a person: Unsound in mind, slightly insane, crazy.

...for which the first relevant citation is...

1610 Bp. J. Hall Common Apol. against Brownists xxvii. 68
That which this man was wont so oft to obiect to his brother (a crack't braine).

As to the specific conjunction crackpot, I think it's worth noting that -pot is appended to various other "fused words" applied (usually, negatively) to people - moneypots, honeypot, fusspot, tosspot.

I think it's fairly obvious that crack't/cracked in this context is just a slightly more figurative way of saying broken, not functioning properly. Per @Oldcat's comment below, additional -pot component could be interpreted as head, brain, skull. Or it may be simply added for prosody (or by association with related forms), and not really add any specific extra meaning.