no flies on one

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, on, one