Q: Am I right in believing that the phrase “plumb loco” is derived from the plumb used to determine the depth of water and a true vertical line? In other words, someone who’s plumb loco would be askew.
A: You’re right that the adverb “plumb” used in this sense is related to the lead plumb bob that’s hung from a line to determine water depth or verticality. But the relationship isn’t quite as straight as a plumb line.
English adopted the noun “plumb” in the 14th century from Anglo-Norman and Old French, but the word is ultimately derived from plumbum, the Latin term for lead, according to John Ayto’s Dictionary of Word Origins.
Interestingly, the word “plumber” is a relative. It originally referred to a worker in lead, but came to mean someone who installs water pipes, which were once made of lead.
Getting back to your question, the Oxford English Dictionary says the adverb “plumb,” meaning vertically, first showed up in English in the early 15th century.
In the early 16th century, the adverb took on the sense of “exactly in a particular direction, position, or alignment; directly, precisely,” according to the OED.
By the end of the century, the adverb was being used in the sense you’re asking about—as an intensifier meaning completely, absolutely, and quite.
The OED’s earliest citation for this usage (with “plumb” spelled “plum”) is from The Misfortunes of Arthur, a 1588 play by Thomas Hughes based on the Arthurian legend:
“The mounting minde that climes the hauty cliftes … Intoxicats the braine with guiddy drifts, Then rowles, and reeles, and falles at length plum ripe.”
Here’s an example, with the modern spelling, from Rudyard Kipling’s 1897 novel Captains Courageous: “You’ve turned up, plain, plumb providential for all concerned.”
Although the OED has many British examples of “plumb” used as an intensifier well into the 20th century, the dictionary describes the usage as “Now chiefly N. Amer. colloq.”
Oxford doesn’t have an entry for “plumb loco,” but it includes the phrase in an 1887 citation for the adjective “loco,” from Outing, an American monthly magazine: “You won’t be able to do nuthin’ with ‘em, sir; they’ll go plumb loco.”
The OED says English borrowed the adjective “loco” in the mid-19th century directly from Spanish. It means mad, insane, or crazy in both languages. The dictionary describes the term as “colloq. orig. U.S. regional (west.).”
Oxford traces the adjective to earlier nouns in Spanish and Portuguese meaning madness, but the editors say the etymology is “uncertain and disputed” beyond that.
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