Phrase " nap-of-the-earth" origins

Nap of the earth, as a military phrase refers to low level contoured fight over the earth surface.
A friend contends that the phrase is older than it usage in the Vietnam war era, usauly pretaining to helicopter tactics.
Have been unable to find the origin of this phrase, and would welcome any help. E-mails welcome.

I've not heard of this phrase, but a number of the meanings of the word "nap" would seem to be relevant. Nap can mean the soft or fuzzy surface of a fabric - for example, the "nap of the cloth" is important to professional snooker/pool players when gauging how to pace their shots, dependant upon whether they're playing with or against the nap. To "nap" something also means to cover it with a sauce.

Both derivations of "nap" in this sense seem to come from the French "nappe", meaning a covering or tablecloth.