no flies on one

there are no flies on (one)

1. One is very active or works at a fast pace. Alludes to the idea that flies do not or cannot settle on fast-moving animals. There are certainly no flies on Judy. She works so fast that the rest of her team can't keep up with her.
2. One is not at all dull or dimwitted; one is exceptionally sharp or shrewd. In this usage, "there are" is often contracted colloquially into "there's." A: "My son keeps trying to get his little sister to do his chores for him, but she sees through his ploys every time." B: "I tell you, there's no flies on her." There are no flies on our boss. He always seems to be one step ahead of what the market is doing.
See also: flies, no, on, there
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

no flies on one

One is wide awake; there is nothing slow or dull about one. For example, She may be new to this field, but there are no flies on her. This slangy expression, which alludes to flies settling on a sluggish animal, was being used in Australia in the 1840s but did not appear in America until the last decades of the 1800s.
See also: flies, no, on, one
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.
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