(In plural) the part of the day (typically) spent at work; (later also) the amount of time spent at work; (in singular) an hour during which work is done.
Origin
Late 17th century.
Definition of working hour in US English:
working hour
noun
(In plural) the part of the day (typically) spent at work; (later also) the amount of time spent at work; (in singular) an hour during which work is done.