When did the term "undercover" begin?

Undercover is a dialect with no formal origin. The original meaning is to fall asleep under the bed, and it is extended to ambush and be an insider.

Lying here means falling asleep, lying down or lying down: ~ falling down. ~ shopping. ~ room. ~ bottom (ambush down to be an insider, such as "~ ~ cross"). ~ slot. ~ dragon (used to refer to an outstanding talent who lives in seclusion or has not yet made his mark). ~ give a little courage (describe hard work and self-motivation, find your ambition back). Hidden dragon ~ tiger.