Calaf , calamus,& halma/halm

>How common is Lat. calamus?

 

Quite common, and in all the senses of harundo.  It occurs more in poetry than prose (perhaps), and the poets may have chosen it over harundo because of its metrical shape, which is much more useful in dactylic verse.  But it also occurs in prose, where that factor wouldn’t apply

 

>Is it a common enough word to have easily been borrowed into Brittonic?

 

That raises all sorts of questions about the nature of borrowing.  How common is “schadenfreude” in German?  But English has borrowed it.    

 

Peter