This phrase, which literally means “to eat a tiger by pretending to be a pig”, refers to what is commonly known in English as hustling. In other words, to gain an advantage by feigning one’s vulnerability.
Apparently this was a common tactic among Chinese hunters. Back in those days, a hunter could not simply stalk a tiger and expect to successfully catch it without employing some level of trickery. As a result, hunters would pretend to be pigs in order to lure a tiger in with the prospect of a juicy porcine treat. At the moment when the tiger lunges for the pig, the hunters would strike back and snatch the tiger up for everyone at the village to enjoy.
Though it seems implausible this would actually work, the phrase is nonetheless commonly used to describe one whose vulnerable demeanor betrays a cunning interior.