Definition
To go unnoticed, especially for a long period of time.
Origins
From military contexts, where aircraft would fly low to avoid being detected by radar.
Also Said As
- slip under the radar
How People Actually Use It
To act sneaky and secretly without other peoples know about it. To act furtively.
"Guy 1 - Have you seen those news about the mobsters of L.A.? Guy 2 - Oh yes, I have. They were finally discovered, working undergrounds. Guy 1 - Haha, they were flying under the radar of our city police, but sooner or later they would be found. Guy 2 - To fly under the radar is no easy deal, here in L.A. ... One hour or other they catch you."
Source: Urban Dictionary