A buttload is a real unit of
measurement or unit of volume that is used for wines, ales, ciders,
and other alcoholic beverages.
A butt is defined to be 2 hogsheads, which in the US is 63 gallons,
so a butt is 126 gallons. A hogshead (corruption of the word oxhead)
traditionally was two barrels and a hogshead of beer or ale is 54
gallons.
This has no relation to boatload, which is "the cargo that a vessel
carries or is capable of carrying". A boatload might be about
54,107,280 gallons. It should also not be confused with butt that
is a target for practicing archery, a part of the anatomy, or any
other of the many definitions for butt.