In Roald Dahl's Matilda, Mr. Wormwood is the father of Matilda and her brother, Michael. He is dishonest, arrogant, obnoxious, and a terrible parent.
He is a used car salesman who makes a living by cheating his customers. He uses sawdust to grease the motors of the cars he sells, so they will appear to run well for a short while. He shamelessly boasts,
Sawdust is one of the great secrets of my success.
He adjusts the odometers of the cars to make it seem like they have less mileage on them than they actually do. He sells stolen car parts and stolen vehicles.
He is also abusive and cruel to his daughter. Instead of being proud of Matilda's impressive abilities and love of reading, Mr. Wormwood is unsupportive and mean. He calls his daughter names regularly and is completely disinterested in her. At the end of the book, he does not hesitate to leave Matilda in the care of her teacher, Miss Honey.
Mr. Wormwood's physical appearance is an outward reflection of the terrible person he is. Dahl likens Mr. Wormwood's appearance to that of a rat. He is described as a
small ratty-looking man whose front teeth stuck out underneath a thin ratty moustache.
Rats are known for being sneaky and evoking feelings of disgust and revulsion. By comparing Mr. Wormwood to a rat, Dahl is further illustrating what an awful person Mr. Wormwood is.