If someone describes you as having No Flies On You they're saying you are quick-witted, alert and active. The idiom's origin can be traced to the cattle ranches of both America and Australia. The first known record of it is from the mid-1800s. The most active cattle and horses would attracted fewer flies than slower, more sluggish animals. The phrase came quickly into wide use and, in the early 20th century, even the Salvation Army popularised it by adopting a hymn entitled "There are no flies on Jesus."