Charles Ellicott Commentary Matthew 20:12

"saying, These last have spent [but] one hour, and thou hast made them equal unto us, who have borne the burden of the day and the scorching heat." — Matthew 20:12 (ASV)

But one hour—Literally, in what was probably the technical language of laborers, worked only one hour.

The burden and heat of the day—The word translated as "heat" is used elsewhere—as in James 1:11 and the Septuagint of Jonah 4:8—for the "burning wind" that often follows the sunrise and makes the labor of the first half of the day harder than that of the latter.