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!
Many, as in, more than a few in quantity.
Here's how you count in the ghetto:
One, two, three, and a shit load.
More than six and less than 29, of any object.
I see a shit load of deer by the woods!
You've got a shit load of marbles in your hand.
The amount of anything it would take to fill YOUR toilet up to breech the water level.
DAMN that is a shit load of SHIT!
Approaching, or in excess of 500. (For gross excess of 500 see: Mega shit load or fuck load.)
In response to seeing a quantity of wads equal to 500: "WOW! That's a shit load of wads."
The ammount of perfect solutions Brandon has sold in a week!
Holy shit! Brandon has sold a shit load of perfect solutions this week,resulting in him getting an ass load of money.