Learn English – How did “at once” get to be used to mean “immediately”

etymologyidioms

How did "at once" become an idiom meaning "do this immediately" or "as soon as possible"?

I just thought of this question after seeing this old UK WWI Propaganda poster in the link below. I glanced over it, and thought it seemed a bit awkward to be used in this way. Which made me curious about how it came to be used in this manner.

WW I poster - "It is far better to face the bullets than to be killed at home by a bomb. Join the army at once & help to stop an air raid. God save the King"

Only way I could think of how this came to be used, is if they were referring to a "timer" starting or something. As if it means "start at the first moment", or "start at the first chance". Maybe, "first" somehow being implemented as "at once"?

Either way, I cant find any references in regards to how "at once" first became used in this manner.

Best Answer

The sixth and final definition of at once in the OED means "immediately, straightway" and is first documented in William Tyndale's Expos. & Notes way back in 1531:

The apostles were clear-eyed, and espied antichrist at once.

It comes from a meaning of "at one time" and other definitions of at once are of things happening at at the same time; or in one heap or body together; or at the same time or simultaneously.

So the "immediately" sense can be similarly thought of as things happening all at the same time, or right now.

Join the army at once & help to stop an air raid

Or:

Join the army right now & help to stop an air raid

You read the poster and then join the army all at the same time, or immediately.