A corruption of the term "shipload" of antiquity, as described the massive cargoes brought into harbor by sea-faring ships. Through association to the perjorative "shit" (itself a corruption of "shite"), the terms "fuck load", "ass load", and "butt load" were all created, the user, unaware of the origins of the term, substituting one curse word interchangeably for another. Over time, the second half of the expression also came to be substituted, most often in cases where "ton" was inserted for "load", whilst still maintaining the previously-replaced derisories as variations of the newly-coined phrase.
In its "proper" form, the phrase would be hyphenated and expressed as a linkage between two words to describe a third, correlated concept (i.e. "shit-load")
Americans commit a shit load of atrocities against the English Language!