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1.
when someone is in a situation where very little or no attention is drawn to them
somehow they managed to stay under the radar for years while skimming all that cash from the bank.
by bungalow bill May 02, 2006
 
2.
Another way of saying underground. the opposite of mainstream. Usually referring to an artist or song that is undiscovered.
Boy: This song is good. Who is it by?

Girl: It's by Bobby Bob, he's under the radar though.
by assyokick April 27, 2011