Probably from the same as basis (through the notion of going down; compare bathos); weight; in the New Testament only, figuratively, a load, abundance, authority -- burden(-some), weight.
KJV: Saying, These last have wrought but one hour, and thou hast made them equal unto us, which have borne the burden and heat of the day.
NASB: saying, 'These last men have worked only one hour, and you have made them equal to us who have borne the burden and the scorching heat of the day.'