Originating from the spanish word for a food, a quesadilla is a flour tortilla folded in half around a savory filling, as of cheese or beans, then fried or toasted. Many middle-americans and Caucasians adopted this way of pronoucing the the word quesadilla (pronounced Kas-Ah-Dee-Ah) through simple ignorance and by having no exposure to any other cultures besides their own. This was one of the most famous words spoken by the white, small town, redneck grandmother in the movie Napoleon Dynamite
From Napoleon Dynamite:
"Grandma there's no food"
"Make yourself a dang casadilla!"