The adverb hands down means easily and decisively, especially in to win hands down.
It originated in horse racing: a jockey who is winning comfortably is able to lower his hands and relax his hold on the reins; the earliest mention of this that I have found is from Bell’s Life in London, and Sporting Chronicle (London) of 17th June 1832:
Wednesday—[…] The Wilton Stakes […] was won by Mr. Beardsworth’s Independence, 6 years (Darling), beating The Cardinal (5 years) […]. Buffoon made the running, Cardinal close up; Independence followed to the hill top, when he challenged the leading horses, kept close to The Cardinal, and won with hands down.
The earliest figurative use of hands down that I have found is from Forests and Open Spaces around London, published in Companion to the Almanac; or Year-Book of General Information for 1866, appended to The British Almanac of the Society for the Diffusion of Useful Knowledge, for the year of our Lord 1866 (London):
It is a contest between Red-tape and the House of Commons, in which Red-tape will win, hands down.
The adverb hands down came to be also used to express emphatic assertion of a superlative, in the sense without doubt, by a long way. This usage seems to have originated in American English; the earliest occurrence that I have found is from The Buffalo Enquirer (Buffalo, New York) of 1st November 1904:
In a show that is not above the average of attractions on the wheel Joe J. Sullivan stands out the bright and shining light in Sam Devere’s Own Company, playing this week at the Lafayette. Sullivan is hands down the best “Rube,” who has played at the Lafayette this season, and that is saying something considering that not a show that plays there but carries its alleged “Rube” comedian.
The adverb is also used to mean submissively, without protest. The earliest instance that I have found is from The Sheffield and Rotherham Independent (Yorkshire) of 7th April 1896, which reported on “the unhappy relationship which continues to exist between the Rector (Rev. A. E. Sorby), and the Vicar (Rev. B. T. Barnes)” at Darfield:
He (the rector) had before witnesses, offered the vicar, in writing, several times, to have the question of rights submitted to a board of arbitration, such as two great ecclesiastical lawyers. He had promised to loyally abide by such decision, but the vicar had refused from time to time to do that. He felt that, as rector of the parish, he had done everything that a gentleman and a Christian could do to solve these difficulties, and the only course open to him now seemed to be this—that he must “surrender, hands down, to the vicar.”