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sundae [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]
float (n.): apparently an early Middle English merger of three related Old English nouns, flota "boat, fleet," flote "troop, flock," flot "body of water, sea;" all from the source of float (v.). The early senses were the now-mostly-obsolete ones of the Old English words: "state of floating" (early 12c.), "swimming" (mid-13c.); "a fleet of ships; a company or troop" (c. 1300); "a stream, river" (early 14c.). From c. 1300 as an attachment for buoyancy on a fishing line or net; early 14c. as "raft." Meaning "platform on wheels used for displays in parades, etc." is from 1888, probably from earlier sense of "flat-bottomed boat" (1550s). As a type of fountain drink, by 1915.
Float.--An ade upon the top of which is floated a layer of grape juice, ginger ale, or in some cases a disher of fruit sherbet or ice cream. In the latter case it would be known as a "sherbet float" or an "ice-cream float." ["The Dispenser's Formulary: Or, Soda Water Guide," New York, 1915]

Few soda water dispensers know what is meant by a "Float Ice Cream Soda." This is not strange since the term is a coined one. By a "float ice cream soda" is meant a soda with the ice cream floating on top, thus making a most inviting appearance and impressing the customer that you are liberal with your ice cream, when you are not really giving any more than the fellow that mixes his ice cream "out of sight." ["The Spatula," Boston, July, 1908]
freezer (n.): 1847 as the name of a type of large tin can used in ice-cream manufacture; from freeze (v.) + -er (1). As a household appliance, from 1945. Freezer burn attested from 1929.
meltdown (n.): by 1937 in the ice-cream industry; by 1956 in reference to a nuclear reactor, from verbal phrase, from melt (v.) + down (adv.). Metaphoric extension since 1979.
softy (n.): also softie, 1863, "silly person," from soft (adj.) + -y (3). Meaning "soft-hearted person" is from 1886; that of "weak, unmanly or effeminate man" is from 1895. The Mister Softee soft ice-cream operation began in Philadelphia, U.S., in 1956.