sundaeyoudaoicibaDictYouDict[sundae 词源字典]
sundae: [19] The ice-cream concoction known as a ‘sundae’ first appeared on the scene in America, at the end of the 19th century. It is generally agreed that its name is an alteration of Sunday, but the why and the how remain unclear. As far as the change in spelling is concerned (sundi was another early variant), it seems plausible that it was intended to placate those who might have been offended by naming a dessert after the Lord’s day.

But why Sunday? Perhaps, it has been speculated, because ice cream was a dessert that could be eaten on Sunday without offending propriety; or because it was a special sort of ice cream that could be sold on Sundays without breaking the Sabbath trading laws; or because it was made of assorted odds and ends of ice cream left over from Sunday. The jury is still out.
[sundae etymology, sundae origin, 英语词源]
sundae (n.)youdaoicibaDictYouDict
1897, American English, thought to be an alteration of Sunday, perhaps re-spelled in deference to religious feelings; but the reason for the name is uncertain; perhaps "ice cream left over from Sunday, on sale later." For a fuller account of the speculations, see H.L. Mencken, "The American Language," Supplement I (1945), pp.376-7.