shellac is a wood finish. It is made of a kind of frass.

"Frass" is the excrement or other debris of insects. "Lac" is neither.

Here is a description of "lac" which is a type of resin [not a frass] used in the production of shellac:

"A secretion is exuded from their bodies, which is in essence a protective covering to prevent an attack by predators. This secretion forms hard resinous layers that completely cover their bodies leaving small anal and breathing openings. The insects mature into adults under this protective layer and both the male and female larvae become sexually mature in about eight weeks."

http://www.vegansociety.com/html/animals/exploitation/shellac.php

BTW "shellac" without the "k" is perfectly acceptable. In fact, "shellac" is the usual spelling. The "k" is required with the verb but it is optional with the noun.