Get the upper hand
Take a dominant position.
Various suggestions have been made as to the origin of 'get the upper hand' (or 'take the upper hand'). Prominent amongst those is that the phrase originated in American playgrounds, in the way that children select sides for impromptu baseball games. The method is for one team captain to grab the bat at the bottom, then the other captain takes hold above the first's hand and they progress hand over hand along the bat until the top is reached - the one left holding the bat having the 'upper hand' and getting first choice of player for their team.
A second theory is that the person whose hand is uppermost when a couple hold hands literally 'takes the upper hand' and is the dominant partner. Thomas Macaulay's History of England, 1848, contains this text:
Then several meetings were spent in settling how many carriages, how many horses, how many lacqueys, how many pages, each minister should be titled to bring to Ryswick; whether the serving men should carry canes; whether they should wear swords; whether they should have pistols in their holsters; who should take the upper hand in the public walks, and whose carriage should break the way in the streets."
It would be incorrect to assume that 'upper hand' was coined as a direct and literal reference to hands, and that isn't what Macaulay was referring to. The earliest citations of the phrase, which predate the above by some centuries, put the emphasis on 'upper' rather than 'hand' and they indicate that 'upper hand' simply meant 'above', either higher in social status or physically above. 'Hand' was a synonym for possession, as in 'in hand', 'hand over' etc. The Romans used the Latin word 'manus', which translates into English as 'hand' to denote the power a husband had over his wife.
The first example of 'upper hand' that I have found in print supports that interpretation of the phrase. This is from The English And Scottish Popular Ballads, collected by Francis Child and published by him in 1882. The ballad in question is Little Musgrave and Lady Barnard, which Child believed to have originated around 1600:
"A grave, a grave," Lord Barnard cryd,
" ‘To put these lovers in;
But lay my lady on the upper hand,
For she came of the better kin."