If you add hyphens, this idiom can be used as an adjective.

Kill two birds with one stone

This common expression means to achieve two goals with only one action. 

Triple threat

This generally refers to a person or thing that has three different skills or qualities. It was originally used in the entertainment industry, particularly musicals, for a performer who could sing, dance and act.

At the eleventh hour

This is the expression we introduced at the beginning of this article. It refers to something happening at the last possible moment. It originates from a story in the Bible about a landowner who hires workers just before it’s too late.

Fifth wheel

A "fifth wheel" is something unnecessary or extra. This expression comes from the idea that a fifth wheel on a carriage that already has four is not needed.

A thousand times no

We use this expression as a way to refuse something very strongly. It expresses much more determination than simply saying “No.”

Sixth sense

Humans generally have five senses: sight, sound, smell, taste and touch. A “sixth sense,” then, represents intuition, or a special ability to see or understand things that are not clear in the physical world.

This is also the title of a popular thriller from 1999 about a young boy with mysterious abilities.

Catch-22

This expression comes from Joseph Heller's novel Catch-22 from 1961. It refers to a situation or rule that seems to contradict, or disagree with, itself. It is often used when getting a positive result is impossible no matter what kind of action is taken.

On cloud nine

Being "on cloud nine" means experiencing extreme happiness. Its origin is not clear, but it is believed to be related to categories of clouds in American meteorology, the study of weather.

Second nature

If something is “second nature” to you, it means you can do it easily and naturally. In other words, it’s just like an ability you were born with.

Expressions with cultural and historical background

The expressions in this section have a strong connection to the religious and cultural backgrounds they come from.

The third time’s the charm

This expresses the idea that something will be successful when tried for the third time.

Historically, the number 3 has represented completeness or holiness. Christianity has the concept of the Trinity, and in many folktales and stories, 3 plays an important role.

At sixes and sevens

This is an expression that is more common in British English. It is said to have originated from the idea that rolling a six or seven in a dice game meant players were in a risky situation. These days, it refers to a state of confusion or disorder.

Talk nineteen to the dozen

This is another mostly British expression, originally used in the 19th century to refer to a system for working in coal mines efficiently. It’s now used to mean talking very quickly and energetically.