PIE*pleuの「流れる」「浮く」から。語源的にはfloat、fleetと同じ。 floatから派生して空中に浮く、飛ぶ、飛翔する、など。同時に、小さな昆虫などを指すのにも使われる。
When the catcher sees several fielders running to catch a ball, he should name the one he thinks surest to take it, when the others should not strive to catch the ball on the fly, but only, in case of its being missed, take it on the bound. ["The American Boys Book of Sports and Games," New York, 1864]