To be in agreement or have a mutual understanding with someone, sharing similar thoughts or perspectives.
This phrase originates from radio technology of the early 20th century, where radio receivers needed to be tuned to the same frequency or wavelength as the transmitter to receive clear signals. The metaphorical use emerged in the 1920s-1930s as radio became commonplace, extending the technical concept to human communication and understanding.
What's fascinating is that this phrase perfectly captures how communication really works in the brain - our neurons literally synchronize their electrical oscillations when we're truly connecting with someone. Modern neuroscience has shown that people who understand each other well actually do have brain waves that align, making this radio metaphor surprisingly accurate on a biological level.
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