no flies on (one)
One is very active or works at a fast past. 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.
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.