Where did the word "umpteenth" come from, and why is it a "teen", if it is supposed to represent a very large number?
Learn English – Where did the word “umpteenth” come from
etymology
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From Etymonline:
doggone 1851, Amer.Eng., a "fantastic perversion of god-damned" [Weekley]. But Mencken favors the theory that it is "a blend form of dog on it; in fact it is still often used with it following. It is thus a brother to the old English phrase, 'a pox upon it,' but is considerably more decorous."
Others have it derived from the Scottish dagone, or "gone to the dogs," but there's a bit of debate about that.
From my own search, I found several pre-1851 uses. It looks like it may have first appeared in print in a British publication, albeit of an American vernacular dialog, in Adventures in Mexico and the Rocky Mountains by British explorer George Ruxton, 1847 (date check):
It subsequently appeard three times in 1848 in Blackwood's Edinburgh Magazine, in articles written by Ruxton while living in America.
If you've ever had an electric shock, you're well aware of the jolting effects of a live wire.
It didn't take too long for the term live wire to enter the vernacular:
Probably everybody likes to be known as a "live wire." The First National Bank, of Ashland, Ala. thinks so for they recently used an advertisement which read as follows: "Every live wire has a bank account. Are you a live wire?" (The Bankers magazine, 1921).
If, however, you accuse yourself of being a live wire and capable of acting without the sound of a gong or the beck of a boss (from a 1922 ad)
More recently, from the realm of music:
Don't touch me, I'm a real live wire (Talking Heads, Psycho Killer)
I'm guessing that wired evolved from the term live wire, which connotes being energetic, full of verve, and ready to go.
Best Answer
Umpteenth comes from umpty, meaning an indefinite number. Etymology Online says "umpty" is derived from "Morse code slang for "dash," influenced by association with numerals such as twenty, thirty, etc."
And I think the "teenth" insert is to make the "th" easier to pronounce for the ordinal formation, but it might be also to add emphasis by way of lengthening the word. Also, it might be because of the teen/ten connection I mentioned in my comment above: an indefinite number of tens.
And this source suggests that Morse code slang word "ump" is imitative in origin, so I gather that means that that's what a dash sounds like in Morse code.
It occurred to me that this means something like _th times, then, or the equivalent of nth times (to the nth degree?)—although a single dash is Morse code for T.