In a well known Shabbath song, there is a line that goes, "Kodesh Hee Lachem Shabbath HaMalkah." Roughly, this translates to, "Holy is she to you, the Sabbath Queen." Oddly, however, the modifier Kodesh (holy) is written in masculine form. I know that frequently people say Shabbath Kodesh, which modifies the female word Sabbath with the masculine word holy, but that I've always chalked up to colloquial error or something. But here, Queen is pretty explicitly feminine. So why is the modifier masculine?
As Alex explained, kodesh is not an adjective. It also has no feminine form. It most likely means "something holy", which describes the sabbath queen very nicely.