/ ˈsʌndeɪ; NAmE ; -di/ noun
a cold dessert (= a sweet dish) of ice cream covered with a sweet sauce, nuts, pieces of fruit, etc., usually served in a tall glass
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WORD ORIGIN
late 19th cent. (originally US): perhaps an alteration of Sunday , either because the dish was made with ice cream left over from Sunday and sold cheaply on the Monday, or because it was sold only on Sundays, a practice devised (according to some accounts) to circumvent Sunday legislation.