I took a quick look into Foster's Complete Guide to Middle-Earth as well as my big German Tolkien Encyclopedia and found no reference to 'lassaraukar' in either, so I would assume that they are indeed an invention and do not originate in Tolkien's writings.
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'Mercy!' cried Gandalf. 'If the giving of information is to be the cure of your inquisitiveness, I shall spend all the rest of my days in answering you. What more do you want to know?' 'The whole history of Middle-earth.. .'
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