off the beam {adv.} or {adj. phr.} 1. (Of an airplane) Not in the radio beam that marks the path to follow between airports; flying in the wrong direction. * /A radio signal tells the pilot of an airplane when his plane is off the beam./ 2. {slang} Wrong; mistaken. * /Maud was off the beam when she said that the girls didn't like her./ Contrast: ON THE BEAM.