Here is a list of number idioms.

Once and for all

If something happens once and for all, it happens permanently.

We will cut off all ties with her once and for all.

Once bitten, twice shy

This is a proverb. It suggests that a person is likely to avoid people or things that have hurt them in the past.

Once in a blue moon

If something happens once in a blue moon, it happens rarely.

I meet my cousins only once in a blue moon.

Once in a lifetime

If something happens once in a lifetime, it happens only once during your life.

This is a once in a lifetime opportunity. Don’t miss it.

Once upon a time

This is an expression that typically goes at the beginning of a fairy tale. It is used to suggest that we are talking about something that happened long time ago.

Opportunity knocks but once

This is a proverb. It means that you will get only one chance to do something.

Second banana

The second banana is someone who plays a subordinate role.

Second best

If something is the second best, it is not as good as the one you wanted.

Second childhood

An expression used to refer to a person’s twilight years when they show reduced mental capacity.

Second fiddle

This expression is used to refer to someone who plays a subordinate role.

Second guess

To second guess is to predict what another person will do.

Secondhand

This expression is used to refer to a product that was previously owned.

Second nature

Used to refer to a particular trait that you seem to have been born with