Dissembling is a tricky way to say "deceiving." If you're good at pretending and lying, you're an expert at dissembling.
Dissembling is a fancy word for being tricky, slippery, and deceitful. This word isn't just for people who lie and commit fraud: it specifically has to do with pretending. If you act like you're sorry you ate the last cookie, but you're secretly happy about it, you're dissembling. A good con man needs to be a master of dissembling. Politicians get accused of dissembling all the time because their speeches sound so phony.