From BBC:

Gerald Holtom, a designer and former World War II conscientious objector from West London, persuaded DAC [Direct Action Committee Against Nuclear War] that their aims would have greater impact if they were conveyed in a visual image. The “Ban the Bomb” symbol was born.

He considered using a Christian cross motif but, instead, settled on using letters from the semaphore - or flag-signalling - alphabet, super-imposing N (uclear) on D (isarmament) and placing them within a circle symbolising Earth.

[...]

As the sign became a badge of the burgeoning hippie movement of the late 1960s, the hippies' critics scornfully compared it to a chicken footprint, and drew parallels with the runic letter indicating death.

In 1970, the conservative John Birch Society published pamphlets likening the sign to a Satanic symbol of an upside-down, “broken” cross.

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