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.
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.