Does anyone know the origin of the phrase, "Nap of the earth?" It is used to describe flight that skims the landscape in order to approach a target undetected and to avoid enemy fire. It was used in US Army aviation as early as 1967.
The ordinary meaning of "nap" is to doze off during the day. The secondary meaning is "to be offguard". Hence the phrase "Catch them napping" . Flying by the "nap of the earth" refers to the practice of flying low to the earth (undetected by radar) to catch the enemy by surprise (or "napping").