slick as a whistle
Very-smooth and neat; also, smoothly, quickly, easily. For example, That salesman is as slick as a whistle, or The fence post went in place slick as a whistle. The allusion in this simile, first recorded in 1830, is not totally clear, but presumably it refers either to the ease of producing a whistle or to its clear tone.
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.