slick as a whistle - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.