upper sixth

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Also see: sixth

Concise Oxford English Dictionary © 2008 Oxford University Press:
upper1

▶adjective

• 1 situated above another part.

  ■ higher in position or status.

  ■ (often Upper) Geology & Archaeology denoting a younger (and hence typically shallower) part of a stratigraphic or archaeological division: the Upper Palaeolithic.

• 2 situated on higher ground.

• 3 [in place names] situated to the north.

▶noun the part of a boot or shoe above the sole.

– phrases
have (or gain) the upper hand have (or gain) an advantage or control.
on one's uppers informal extremely short of money.
the upper crust informal the upper classes.

– origin ME: from the adj. up + -er2.

Concise Oxford English Dictionary © 2008 Oxford University Press:
upper2

▶noun informal a stimulating drug, especially an amphetamine.

– origin 1960s: from the verb up + -er1.