I think "ain't no flies" falls closer to Mrrzy's understanding of it. A friend of mine (now in her upper 80's) always used to use the remark that there "weren't any flies on someone-or-other" to mean they were agile of mind, not easily fooled. But she lived on a dairy farm, so the saying could also easily mean the general level of physical activity that keeps pests away (the twitching of one's tail, if one is a horse or dairy cow).

SRS