A Mawworm is a hypocrite, and mawwormism is sanctimonious hypocrisy.
Any bookworms, rather than mawworms, might already be a step ahead of us here, as there’s a literary connection behind this one. In 1768, the Irish writer Isaac Bickerstaffe wrote a comic play called The Hypocrite, which was in its own way a retelling of the great French playwright Molière’s play, Tartuffe. Molière’s iconic character of Tartuffe was famously an unpleasant, sanctimonious, religious hypocrite—and so too was the central character in Bickerstaffe’s play, whom he named Mawworm.
The popularity of the play in eighteenth and nineteenth century theatres later prompted a host of related words to emerge in the Victorian vernacular; the Oxford English Dictionary has entries for mawwormy, mawwormish, and mawwormism—the latter dated to 1850.
As for the word mawworm itself? Well, long before Bickerstaffe picked it up, a mawworm was an actual creature. Maw is an old word for the stomach of an animal, and since the sixteenth century the term maw-worm has been used of a clutch of parasitic creatures that can infest the stomach and intestines of animals, and humans. So however way you look at it, this is a fairly unpleasant word.