Why do bombs, alarms, flashbulbs and the like "go off" instead of on?

Aren't those all items you have to set (on) in order to make it go off?
I mean the alarm cannot be on and go off at the same time.
(or something like that)eh.. I mean , there has to be an 'on' and an 'off' to keep things clear.

Perhaps the same off as the start of a horse race ("They're off!") or "We're off to see the wizard": performing a designated function or mission?

Or when someone starts to rage at another, its: "Going off".

I suspect this evolved from the bomb/alarm/flashbulb usage.

I was told years ago that the use of phrases involving a preposition at their end is largely an assimilation of Black American slang from the early part of the 20th Century. Bombs detonate or explode, they don't "go off." Likewise, buildings don't "burn up" or "burn down:" they simply burn, or they may burn completely, or they may be razed by fire. Alarms are armed or disarmed, activated or deactivated, or simply set. If they actually sound an alarm, they've been triggered or they sound. Almost all thoughts that are commonly expressed with these upside-down "prepositional phrases" can be expressed using other words or phrases that we seldom hear any longer. I might be completely wrong about whence these sloppy (and often incomprehensible) expressions came - hopefully one of our linguists will add to this thread, because I've always been curious about this myself.

It's a feature of Germanic languages; German does it too.

Wait... incomprehensible? What's incomprehensible about go off, burn up, burn down?

Othello says: I should make very forges of my cheeks,
That would to cinders burn up modesty,
Did I but speak thy deeds.

Helena (All's Well That Ends Well):
Bless our poor virginity from underminers and
blowers up! Is there no military policy, how
virgins might blow up men?

I was told years ago that the use of phrases involving a preposition at their end is largely an assimilation of Black American slang from the early part of the 20th Century. Bombs detonate or explode, they don't "go off." Likewise, buildings don't "burn up" or "burn down:" they simply burn, or they may burn completely, or they may be razed by fire. Alarms are armed or disarmed, activated or deactivated, or simply set.

This is preposterous. The use of verbal particles (prepositions, preverbs, what have you) has been a part of English for at least a 1000 years. In fact most of the Germanic languages have them.

An example from Middle English cited in the OED2: "c1305 in E.E.P. (1862) 4 þe fire sal berne vp sinful man þat haþ misdo."

I think so too. A house can burn and when the fire is put out in time it will not burn down.
When a cigar is getting towards its end it is almost burned up.
When a person has lost all taste of working you call it a burn out. ( We call it the person is burned up )

Only Jeeves would ask: "Is the alarm armed?" Ordinary people ask: "Is the alarm on?"

And so 'For be fire sal berne vup sinful man pat hap misdo'

I don't doubt that these type of phrases existed before the early 20th Century, but in reading older works, particularly dialogue in older works, they are scarce. The English in which I've grown up is absolutely saturated with them. As for Jeeves, that is exactly my point. Well spoken people of a certain age were not likely to use these phrases, whereas today they are used ubiquitously.

I'm not defending the claim that their common use derives from black slang - that's only something I read somewhere, years ago - but there must be an explanation for the explosive popularity of the form in the 20th Century.

We've gone from "Let's give Bob a warm welcome" to "Give it up for Bob."

I just noticed this:

"Wait... incomprehensible? What's incomprehensible about go off, burn up, burn down?

Nothing at all to people who have grown up with them, but they are often incomprehensible (or at least illogical) to people learning English! Which does it do? Burn up or burn down? Shouldn't those have opposite meanings? I had an acquaintance from Hungary who was mystified by the phrase "hand out," in the sense of documents distributed to attendees at a meeting. Makes perfect sense to me, but it was mumbo-jumbo to him.

but in reading older works, particularly dialogue in older works, they are scarce

In older works the difference between spoken and written language my have been bigger. But yes, if I read: "Give it up for Bob" I wouldn't know what it means. But in the context of a meeting and in spoken language I would understand.

Prepositions are the trickiest thing in foreign languages. I will always be marked a foreigner by little mistakes in that field. Just as we can find out foreigners, even those who speak the language very well and have lived here for decades by those tiny two- or three letter words.

they are often incomprehensible (or at least illogical) to people learning English! Which does it do? Burn up or burn down?

They're idiomatic (in the sense that you have to learn their meaning because it is not immediately apparent from the various parts)) and they're phrasal verbs, and as goofy and I have pointed out, they're something that goes back (beyond) the earliest recorded English. All language is incomprehensible to people just learning it. The solution is you have to memorize their meaning along with the verb. I am constantly confounded by idiomatic phrases in Japanese, so much so that I bought a dictionary of idioms.

That being said, if you study the verbal particles (and I own a whole bunch of monographs and books on them, going back to the late 19th century) in English can can see that they are systematic at least.

But it's so easy to unsystematically confuse them.

to unsystematically confuse them

Actually, it's just getting the system wrong. I consistently "conjugate" verbs incorrectly for Japanese, but my errant forms are correct for the language 500 years ago. The intervening years have seen some phonological changes that seem less systematic at first, but I am starting to get the hang of the "system". But, yes, I agree with you about the pesky left-over bits of languages, i.e., the adverby, prepositiony, particley morphemes.

And tying this in with a thread down south of the fold, I have always wanted to see a study done on the use of prepositions in Esperanto with special attention to the speaker/writer's native tongue.

This could be the Recency Illusion.

imagine my surprise (not) to find that there is such a thing: Recency Illusion
-joe (imaginary) friday

great name for a band.

Ah, yes; often observed in young adults, I have noticed.

Took the words right off my fingertips.