The story
From butt + load. Butt in this context may be possibly one or both of: * butt (“large wooden cask”) (Etymology 3) * butt (“two-wheeled cart”) (Etymology 5) Alternatively, the term may either be a corruption of English boatload or have been influenced by that term (except for the specific, West Country dialect sense). All senses above also synchronically reanalyzed as buttocksful.
Etymology from Wiktionary, CC BY-SA 4.0.