The bottom or base of something, particularly the bottom of a bottle or glass; also can mean the buttocks in Scottish dialect.
From Scottish English 'doup,' possibly from Old Dutch 'doep' or related to Middle Low German terms. The word is primarily Scottish and Northern English dialectal.
The word 'doup' is a great example of how Scottish English preserved older Germanic words that died out in Standard English—while most English speakers use 'bottom,' Scots speakers still use 'doup,' keeping alive a connection to medieval Northern European languages.
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