Why “go off”, as in “alarm went off”?

I was wondering why does something goes off, when it in fact does the opposite

  • bomb goes off – it blows up
  • alarm goes off – it turns on

Why not goes on?

Answer

To go off in this sense is related to the expression to set off, meaning to start or to be started. It implies that the subject was in a state of rest, then moved off from that state into action. The phrasal verb to go on already has the meaning of to continue.

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