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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?

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To "go off" in both contexts simply means to "activate/perform its intended function". Why does this need a bounty to unearth what's going on? – FumbleFingers Feb 19 '12 at 3:43
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@FumbeFingers first, this bounty was to recompense Jon Purdy's answer. Secondly, in my mind, and perhaps hmemcpy's mind, "on" is associated with the start of a process (online, on air), rather than the end of a process (offline, off air). – Theta30 Feb 20 '12 at 18:33

5 Answers

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+50

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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Very nice explanation. – Manoochehr Jan 30 '11 at 20:44
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An alarm goes off when the alarm is "released" or "raised". An alarm isn't just a thing that is on/off, it's a state of alert.

Similarly when a bomb goes off, it has moved from a gentle, resting state to an excited, explosive state.

Goes off is often used for something that suddenly and explosively changes from a resting state to one of vigorous action.

  • "The runners have lined up, the starter raises his pistol... and they're off!"

  • "The gun goes off and everything changes... the world changes... and nothing else really matters." - PattiSue Plumer, runner

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Still, I think you are bound by usage. A logical explanation of why "off" was used rather than any other preposition would have satisfied me more. – Theta30 Feb 18 '12 at 18:55
up vote 2 down vote

While I'm no expert on idiom origin, I'd venture to guess that the phrase is "alarm goes off" because the first alarms were likely those that needed to be wound up. As one winds them up, the mechanical processes inside coil up, around and on top of each other, creating a tension. So when it unwinds...the coils literally come off of one another, causing to tension to be be released as well until finally the coils are all off, and the tension is gone, which allows the other parts of the alarm to vibrate and make noise.

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As an alternative to the other two answers, I think of "going off" as a trap or reaction getting triggered. "The bomb goes off" means that the reaction was triggered. Rest and motion are not really relevant. Same with a "trap going off" — the trap was waiting for a particular event and then *bam* it went off. "Went off" is a phrase for someone exploding in emotion against another and also brings to mind a triggering event.

An alarm fits in this category of words: "The alarm went off." The trigger for an alarm is a particular time and, when the time arrives, *bam* it goes off.

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up vote -2 down vote

It dates from medieval England when households revolved around a fire upon which a pot would be continuously simmering. This was linked to some sort of bell which would chime continuously. If the fire went off, the energy (heat) from the fire would stop the chiming thus causing the alarm. Hence we use the phrase when the alarm goes off!

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Please add your research to this answer. – MετάEd Mar 14 at 14:52

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