1The work of a day; (an amount of) work done on or appropriate to a day.
2"all in a (also his, the,etc.) day's work": (of a task, experience, etc.) accepted or expected as part of a person's normal duty or routine. Often used ironically.
all in the day's work was the more common form in early use; since the mid 20th century, all in a day's work has been more usual.
historical As a unit of area: = "daywork".
Old English. From the genitive of day + work.