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Phrase(s): the icing on the cake
Fig. an extra enhancement. • Oh, wow! A tank full of gas in my new car. That’s icing on the cake! • Your coming home for a few days was the icing on the cake.
McGrawhill's American Idioms And Phrasal Verbs
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Also, frosting on the cake. An additional benefit to something already good. For example, All these letters of congratulation are icing on the cake, or After that beautiful sunrise, the rainbow is just frosting on the cake. This metaphoric expression alludes to the sweet creamy coating used to enhance a cake. [Mid-1900s]
American Heritage Idioms