Asleep at the switch - an American idiom

America's English idiom

 American idiom:

“Asleep at the switch”

Meaning: not attending to one’s job or failing to react quickly; not being alert.

Origin: From the early days of railroad (mid 1800’s).  One of a trainman’s duties was to switch a train from one track to another at exactly the right moment.  If they fell asleep at the switch, the train might go off the rails and crash.  

America's English