there are no flies on someone

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.

there are no flies on someone

If you say there are no flies on someone, you mean that they are quick to understand a situation and are not easily deceived. Oh, he was a smart one, all right. There were no flies on him.
See also: flies, no, on, someone, there
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012

there are no flies on —

the person mentioned is very quick and astute.
Early instances of this expression suggest that it originated with reference to cattle who were so active that no flies settled on them. The phrase was noted in the mid 19th century as being very common in Australia as a general expression of approbation. In the USA it could also be used to convey that the person in question was of superior breeding or behaved honestly.
See also: flies, no, on, there
Farlex Partner Idioms Dictionary © Farlex 2017
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