It's not a unit per se but an actual size of liquor casks, equivalent to two barrels (also a standard unit in England but is the generic term for casks in US) o

It's not a unit per se but an actual size of liquor casks, equivalent to two barrels (also a standard unit in England but is the generic term for casks in US) or fourth (1/4) of a tun (the largest liquor cask defined), although you can use them all as units. Fun fact: hogsheads are abbreviated as hhds :)