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This question already has an answer here:

Why is the initial letter of some of the words like pneumonia, and psychology not pronounced?

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It is an English phonetic adaptation of Greek words:

There are a number of Greek onset clusters imported into English:

'gn-' as in 'gnostic'

'pn-' as in 'pneumonia'

'pt-' as in 'pterodactyl'

'ps-' as in 'psychology'

  • The 'g' and 'p' are not silent in Greek. But /gn/, /pn/, /pt/, /ps/ are not legal onsets in English phonology. English speakers naturally dropped the first sound to make it more compatible with English.

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