To "Open a Can of Worms” means to start something slated to be difficult and not in one’s control. "Open a Can of Worms” is a well-known phrase that means that one is getting into a more complex area which may have deeper implications. In simple conversational terms it would mean to broach a topic that you had no intentions of going deep into. In a discussion to "Open a Can of Worms” would mean introducing a new set of complications to the problem. The information in the “can of worms’ is regarded as confusing, difficult and wriggly which only further increases the problem.
The phrase is essentially of American or Canadian origin which came to be used in the 1950s. It was common for bait shops to stock cans of worms for fishermen who always found it difficult to close a can of worms once it was opened. “Can of worms” is a noun phrase used in the singular while the plural form is “cans of worms.” The phrase “Open a Can of Worms” is used idiomatically to refer to a complex or troublesome situation that has arisen due to a certain action or decision taken that only increases the existing problems. The phrase “kettle of fish” or the metaphor “pandora’s box” is used synonymously with the phrase “can of worms”
To "Open a Can of Worms” would mean to reveal a messy situation that would be exposed to everyone. Taken literally a can of worms would contain numerous wriggling worms which would be squirming all over the place when opened. It would be a difficult task to get all the worms back in the can. When applied to life, it means that many times one says things which cannot be reversed and gets opened like the can of worms impacting situations and people. The idiom to "Open a Can of Worms" is a metaphor for creating, unintentionally, a number of problems while resolving to solve the main problem. It is obvious then that the situation that is created by the can of worms is going to be unpleasant and troublesome.