What’s the etymology of the phrase “once and for all”? 

What is an idiomatic expression?

Idiomatic expressions are a type of informal English that have a meaning different from the meaning of the words in the expression.

Meanings:

1. finally; decisively; conclusively
2. for the last time
3. completely, for good

Let’s look at some real examples!

• He should act once and for all. 
• This is not an issue that was settled once and for all.
• It would all be a huge waste of time and money and would prove once and for all that Britain was no longer top dog.

Nearby words:

• once again: It happens again.
• once and again: occasionally.
• once bitten, twice shy: when you want to indicate that someone will not do something a second time because they had a bad experience the first time they did it.
• once in a blue moon: very rarely; almost never
• once in a while: it happens sometimes, but not very often. Occasionally; now and then.

Some examples here:

• Now the two women are going into battle once again.
• I forgot to book an appointment once again. 
• They meet for lunch once and again. 
• Sales like this only happen once in a blue moon.

Now, it comes the hardest: Get ready and start incorporating this new expression in your day-to-day communications. Fingers crossed and good luck!