I got to that first link through Metafilter, where many people are reporting on whatever word it was that they thought of seeing in lots of European languages.
I was fascinated to see that "junta" as a translation of "knuckle" (in Portugal, but not Spain). Thinking about the English word "junta" — influenced by all that thinking about knuckles in a physical attack — I picture a punch, but the English word "junta" relates to government by a committee. A committee? What is the connection between committees and knuckles?
It's right there in the word "junta." Look at it.
The word is joint. People join together to form a committee and a knuckle is where bones join — a joint.
So why did marijuana cigarettes ever get called "joints"? One guess is "the notion of something often smoked in common." It's a joint cigarette, a shared cigarette. To smoke one, you form a committee, or "choom gang." But it might be connected to the use of the word to refer to "a complete hypodermic outfit consisting of syringe and needles" (used in 1935 by A. J. Pollock in "Underworld Speaks").
Also from that last link, there's the slang use of joint to refer to a place, "especially one where persons meet for shady activities... perhaps on the notion of a side-room, one 'joined' to a main room." Hmm. If people meet there, they join together there. It seems to me like an obvious leap from a verb to a noun. We join'd at the joint. The (unlinkable) OED has many helpful quotations for understanding this meaning of "joint." Here are a few:
1821 Real Life in Ireland xvii. 199, I had my education at the boarding-school of Phelim Firebrass..; and when I slipt the joint, and fang'd the arm, he strengthened the sinews.
1904 Sun (N.Y.) 6 Mar. 7/4 Of course, there are no saloons in Kansas; no one would dream of calling them by that name. They are all ‘joints’, whether the drinks are passed over a polished counter by a white aproned attendant, or shoved through a hole in the wall by a dirty fist.
a1922 T. S. Eliot Waste Land Drafts (1971) 59 line 50 So the men..thought Of home, and dollars, and the pleasant violin At Marm Brown's joint, and the girls and gin.
1934 P. G. Wodehouse Right ho, Jeeves xiv. 167 Hanging out in a joint called Kingham Manor.