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Why is the term lightoff (or light-off) used to describe the moment an engine ignite/is started ? I'm wondering why the suffix 'off' is used in an 'ON' sense (start, ignition) ? Is there a etymologycal reason, or an historical reason/origin for this use ?

I found this word specifically in aeronautical applications.

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  • $\begingroup$ Just a follow up question... could it not refer to an annunciator light that extinguishes when the engine starts. I can only think of Oil Pressure warning, but I'm sure other lights go to an "off" state? $\endgroup$
    – Greg Woods
    Commented May 14 at 14:00
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    $\begingroup$ @GregWoods The aircraft I've flown all indicate the ignition with a rise in a temperature, either Turbine Inlet or Exhaust Gas temperature. Other lights may extinguish either before or after ignition as the engine spins up, but the fact of ignition is observed via the swing in the gauge, not a light per se. I think the top comment above is more to the point. (And I agree it could become a good answer.) $\endgroup$
    – Ralph J
    Commented May 14 at 17:32
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    $\begingroup$ Note: there are a stack exchange about English (etymology, etc.). I think the question may be more relevant there. The links are on the top right icon (on the banner) $\endgroup$ Commented May 15 at 7:52
  • $\begingroup$ It's English Language & Usage, @GiacomoCatenazzi (The shortcut for that is to type [english.se] and it's expanded into the full site name as a link.) $\endgroup$
    – FreeMan
    Commented May 15 at 17:11
  • $\begingroup$ @GiacomoCatenazzi I've also posted my question on this SE, see link $\endgroup$
    – Vincent
    Commented May 17 at 6:19

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It's one of those quirks of the English language. From the Cambridge dictionary - use of "off" to start a process:

"To begin by doing something, or to make something begin by doing something:

  • She started off the meeting with the monthly sales report.

  • I'd like to start off by thanking you all for coming today."

Another example: To make a firecracker explode, you "set it off".

So, literally, it's the commencement of engine operation by the lighting of the flame in the burner can.

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This would be best answered on English Language and Usage, but I see they already suggested to use Aviation. So this answer.

Off has several meanings, including distance or suddenness and being out. But as a noun it means the beginning. From the off means from the beginning.

Some phrasal verbs also use off in the sense start/launch/trigger/commence, including to light off:

To light off: To set fire to (fireworks or the like) in order to cause an explosion, usually for the purpose of entertainment. The kids are lighting off firecrackers in honor of Chinese New Year. Source.

The light off is the instant the engine is ignited. You'll find the same intent in other phrasal verbs like take off, lift off, start off, make off, set off and the derived nouns. Not forgetting the usual kickoff meeting every project starts with, which has a sense similar to light off.

I'm wondering why the suffix 'off' is used in an 'ON' sense (start, ignition)?

Note the different uses of off:

  • To light + off: The phrasal verb to light off, meaning to set fire to something, to ignite something.

  • To switch + off: Change the state of something to off.

As an example of off used to define some point in time: Liftoff is a term used for a spacecraft takeoff. For the US Shuttle liftoff meant the vehicle top reaching the top of the launch tower. Before this point the count down was negative, after it was positive: T-10s, liftoff, T+3s, etc.

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  • $\begingroup$ Thanks for your answer. As a non native english speaker, I generally understand "off" as not working. In your examples take off, lift off, move off... I understand it as "away from", so it's comprehensible. But it's hard for me to realize when off means ON. $\endgroup$
    – Vincent
    Commented May 17 at 6:32
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    $\begingroup$ @Vincent: As explained in Wikipedia, a phrasal verb is idiomatic, the meaning cannot be inferred from the two words. Think about back up, which has different idiomatic meanings of which: move in reverse for a car and block for a drain. Other examples look after (take care), turn up (appear), tell off (reprimand). If you happen to be a French speaker, passer outre, or compter sur are constructions similar to English/German phrasal verbs. $\endgroup$
    – mins
    Commented May 17 at 8:04

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