So he sweated and fired up and watched the glass fearfully (with an impromptu charm, made of rags, tied to his arm, and a piece of polished bone, as big as a watch, stuck flatways through his lower lip), while the wooded banks slipped past us slowly, the short noise was left behind, the interminable miles of silence—and we crept on, towards Kurtz.
There are no more uses of "impromptu" in the book.
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In one picture, which seems like the most recent but still looks a few years dated, his dad has his arms around the two boys, and he’s squeezing them together for an impromptu photo.
Colleen Hoover -- Slammed
I hesitate for a split second, long enough to place the tune—Frank Sinatra, "New York, New York"—and say a prayer of thanks that the Pugs aren’t witness to my mom’s impromptu performance.