c. 1300, "belonging to an elevated region;" late 14c., "pertaining to the topmost part of an object;" originally the comparative of up (adj.). As "chief, superior" from mid-15c.
In reference to peoples, etc., "occupying more elevated ground" by 1610s, but often also this means "occupying an interior district" (compare high (adj.); High German being that spoken in the upland regions of Germany which also are most interior).
Upper hand "advantage" is late 15c., perhaps from wrestling (get the over-hand in the same sense is from early 14c.); lower hand "condition of having lost or failed to win superiority" (1690s) is rare.
Upper crust is attested from mid-15c. in reference to the top crust of a loaf of bread, 1836 in reference to the higher circles of society. Upper ten thousand (1844) was common mid-19c. for "the wealthier and more aristocratic part of a large community;" sometimes upper ten for short, hence uppertendom.
Upper middle class (adj.) is recorded from 1835. The college upperclassman is so called by 1871. Stiff upper lip, figurative of courage and struggle against despondency, is by 1833. For upper-case for capital letters (1862) see case (n.2).
Similar formation in Middle Dutch upper, Dutch opper, Low German upper, Norwegian yppare. Also as an adverb in Middle English, "higher":
As the sonne clymbith upper and upper, so goth his nadir downer and downer. [Chaucer, Treatise on the Astrolabe]