I'm learning the English language, and while reading Merriam-Webster I often see common words with additional "obsolete" and "archaic" descriptions added to their definitions. 

When should I use them, should I use them at all, and what's the difference between these descriptions? Also, should I spend time to remember these archaic and obsolete meanings? 


Question: Any omitted word in this sentence?
Answer: Yes, "might have been poured concrete" does mean that he was immobile like a poured-concrete cast. 

The sentence could use a comma:


  After he hit it, he might ...


Also the part “been poured concrete” seems odd. I was half expecting it to end “... he might have been poured out and shaken up for all he could remember.”  In other words, coming after been, I tended to parse poured as a verb at first; its use as an adjective threw me briefly. 

But I can't think how one would improve that except by being more verbose, as in:


  ... have been made of poured concrete ...


That's two missing words, rather than one, and it seems to weaken the directness of the author's style.