Shellack is a centuries-old spelling of the word referring to the broadly useful substance secreted by the lac bug. Shellac, however, is by far more common in today’s English. It has been the preferred form since the 19th century, and shellack now hardly appears at all.
The k is preserved in the inflected forms, shellacking and shellacking.
Shellac now doubles as a figurative verb, meaning to beat or to defeat. This use came about in the United States in the middle 20th century, but it has spread throughout the English-speaking world to varying degrees. The verb is commonly embedded in the present-participial form; a shellacking is an overwhelming defeat. The word comes up often in sports.