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.

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    Only a thought, so I'll keep to a comment: perhaps the other sense (at once: All at one time; simultaneously: Everything happened at once. AHDEL), probably deriving from at one time, came to have the additional sense at this (one) time, instantly, its accepted other meaning. – Edwin Ashworth Aug 29 '12 at 16:58
  • @EdwinAshworth concur. I'd guess it originated as a qualifiier or response to a command or request "Do this." - "At once", viz. at the same time as you say it. It replaced presently, which signified "not in the future, but now, even as I/you say it." – StoneyB on hiatus Aug 29 '12 at 17:01
  • Hmm. interesting. Good thought. But how could "at one time" become used as "this (one) time" is the question. What is the "(one)" in time that we are referring to? I don't understand how it could have come from meaning "simultaneously". Which is why I have this question, It just seems it doesn't fit. – insomnia Aug 29 '12 at 17:04
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    @EdwinAshworth: I think that's right. Note, though, that this is a switch from the historic meaning of at one time: At one time this was all oak forest. That's an indefinite one, on the road to becoming a/an. The one of all at one time, on the other hand, is a definite, located, single time in the present or near future in which several propositions converge. Later the all gets dropped and at one time changes to at once, which is criterial now. – John Lawler Aug 29 '12 at 17:05
  • okay nice. That makes much more sense. I can definitely see that. :) – insomnia Aug 29 '12 at 17:12

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


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.

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    Perfect. :) Thank you. And if you tweeted my question, on the @StackEnglish, thanks as well! That was nice. But All of the answers and comments here are great. I upvoted all I felt were great explanations and put things into perspective, but I choose this as the answer because I feel this answers my question dead on. Thanks Hugo, and everyone else. – insomnia Aug 29 '12 at 19:29
  • You're welcome. I think @StackEnglish is owned by Stack Exchange themselves and is triggered by some mysterious "multi-collider" algorithm thingy. – Hugo Aug 29 '12 at 19:58

Since the literal usage means two things happening concurrently, according to Wiktionary, I'd guess that the idiomatic usage assumes that right now is one of the concurrent events.

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