Suddenly and unexpectedly, without warning or apparent cause, describing something that happens completely by surprise.
This phrase, dating from the late 19th century, is a shortened version of 'out of the clear blue sky,' referring to how lightning can strike from an apparently cloudless sky. The 'blue' refers to the clear blue sky from which unexpected events seem to emerge.
The phrase captures our human need to understand causation - we're so uncomfortable with truly random events that we often search for hidden patterns or meanings when something happens 'out of the blue.' Meteorologically, it's worth noting that lightning actually can strike from a clear sky, traveling up to 10 miles from its originating storm cloud.
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