can of worms

IPA/kan ɒv wˈɜːmz/
IPA/kæn ʌv wˈɜːmz/

can of worms — idiom

1. a subject or task that seems simple but, once you begin it, leads to many new an

1.idiomB2
Definition

a subject or task that seems simple but, once you begin it, leads to many new and difficult problems that are hard to stop.

Example

Asking about the missing money opened a real can of worms at the office.

open a can of worms (the most common pattern)

Paloma warned that changing the old rules would be a can of worms.

be a can of worms (subject complement)

Common collocations
  • open a can of worms
  • a whole can of worms
  • a real can of worms
Synonyms
  • Pandora's box

    also means starting something that releases many troubles, but sounds more literary and dramatic

  • minefield

    stresses hidden dangers you must move through carefully, rather than problems that multiply

Grammar Patterns

open a can of worms

be a can of worms

Usage Note

Almost always the object of the verb 'open' — to 'open a can of worms' is to start something that releases many fresh problems. Once opened, the problems are hard to close again, so speakers use it as a warning against starting something messy.

Common Mistakes

This decision will open a box of worms.
This decision will open a can of worms.
the fixed phrase uses 'can', not 'box'; mixing it with 'Pandora's box' breaks the idiom.

Etymology

English

An American expression from the early twentieth century. It pictures a sealed can full of live fishing worms: the moment you take off the lid, the worms wriggle out in every direction and are almost impossible to put back. In the same way, starting certain tasks or subjects lets out problems that quickly spread and cannot easily be controlled again.

First known use

1962

1962, in the meaning defined above