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The words Psychology, Pneumatic etc.  are pronounced with p being silent.  During 16th century or 15th perhaps ,  these words were borrowed from Greek by Latin for denoting Psychological science.   In Greek language the two p and s sounds are pronounced.    In Latin p and s combination does not work.  So p was made silent.  That was the way it was done at that time.

  It was possible that some German people who used the words in the beginning pronounced that way.  It continued.

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