Plural of sundae; a dessert made of ice cream topped with syrup, whipped cream, nuts, fruit, and other toppings.
From 'Sunday'—either because it was originally a special Sunday treat, or because strict Sabbath laws once forbade selling soda water on Sundays so ice cream was served plain instead. The spelling changed to 'sundae' to avoid Sabbath association.
The spelling 'sundae' instead of 'Sunday' is actually a clever attempt to dodge religious controversy—ice cream parlors changed the spelling to avoid offending Sunday-observant customers who thought enjoying desserts violated the Sabbath!
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