[A_Brandybuck]

I doubt as the rest here, that there exists some "lassaraukar" in the original writings.
The word himself seems to be generated from "original" Quenya words and perhaps means "Leaf-demons" . Based on the word "lasse", which means leaf and "rauco" (pl. "raukar"), which means "demon".

I doubt, that the word-creation is original from Tolkien, because the using of the word "lasse" (if it should really mean "leaf") in the word "lassaraukar" fits not the Quenya-grammar. The word should have been "lasseraukar" ("Leaf-demons", nominativus) or "lasseoraukar" ("demons of leaf", genetivus).
Or in plural: "lassiraukar" (nom.) or lassionraukar (gen.).