Dear chrisg, Thank you for the reference but, with due respect to Merriam-Webster, I think their definition is an afterthought. The trouble with being of a certain vintage (or with being an old fart if you like) is that you remember some things very clearly.
To us jazzers in the late 50s, and early 60s the Beats, or Beatniks were old men - that is chaps in their middle to late 20s or even older- who were in to poetry by the likes of Allen Ginsberg, and Lawrence Ferlenghetti (still with us!), and Zen, wore black polo necks and dark glasses and bummed cigarettes off you.
The polo necks and glasses could be confusing: in my local jazz club I once began a conversation with one of these "old men" by saying, "You beatniks - " he held up a hand and said "Whoa man! I'm a hipster."
My reference is the Random House Historical Dictionary of American Slang Vol 1, edited by J.E. Lighter. This attributes the suffix "nik" to the influence of the Sputnik which was launched in 1957, although Yiddish speakers would claim its origin is Slavic.
As for myself, I'm an onthefencenik, and, as you probably now think, chrisg, I am a nudnik too.