An abbreviation whose exact meaning is unclear or context-dependent; possibly stands for 'hogshead' (a large barrel) or other technical terms.
If derived from 'hogshead,' it comes from Old English hog (pig) + head, as the barrel was supposedly large enough to hold a pig; however, HHD as an abbreviation is not standard.
HHD might stand for hogshead—a unit of volume used for wine and other liquids that's so old that most people don't even know what it is anymore, yet it's still technically legal to use.
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