A raincoat or waterproof garment, primarily a Scottish and Northern English dialectal term.
Possibly derived from 'dock' in the sense of a heavy fabric, or from 'Macintosh' (the waterproof material), though the exact origin is uncertain and regional. The term appears to be primarily Scottish.
Regional words for raincoats are amazingly varied across English-speaking areas—'docmac,' 'Macintosh,' 'anorak,' 'cagoule,' and 'brolly' show how local industries and climate created completely different vocabulary for the same object.
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