Where does the phrase rule of thumb originate from? Why the thumb, of all possible body parts?
Learn English – Origin of the “Rule of thumb” phrase
phrase-origin
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Meaning
Etymonline.com says of handy:
Meaning "conveniently accessible" is from 1640s.
To come in handy means something will be handy or useful in the near future, that it will become handy.
Come in handy
The earliest verifiable examples I found in Google Books are both from 1843.
First, in a "Weekly Journal of Gardening" column of The Gardener and Practical Florist:
CELERY, as we always recommend in small gardens, should be planted out at various seasons, and if there be any left in the seed bed, another row will come in handy. Earth up that which is advancing. LETTUCES in the seed bed may be thinned, and those taken out may be planted.
July 15, 1843.
Second, in Tales of the Colonies, or, The Adventures of an Emigrant, Volume 2:
"What have we got here ? a pair of handcuffs ; ah ! these come in handy ; the bushranger won't want handcuffs any more, but they'll do for his mate."
Come handy
We can also find some slightly early uses of the similar to come handy. It was once used similarly and as often to come in handy, but lately has become rarer.
An October 1824 The London Magazine prints a letter from summer 1821:
please your oner,
hoping your oner wont be displeasd at my boldness and I send a little basket of eggs-good fresh eggs-and they were lade by the little black hen that's three yeer ould come Michaelmas eve the day that I send home your oner's shute— and the times are very hard intirely — intirely — plase your oner from
your oner's sarvent to comand,
Timotheus Kinnealy.
the woman hopes the eggs wil come handy to the young mistris out of her confinement. — tuseday mornin.
"I'm down with it" or more often "I'm down with that" is confirmation, acceptance as in "that's fine by me", or commitment. It's originally African American jazz slang from around 1935.
It was popular during the 1990s especially in rap and hip hop, also in the 1970s, and in jazz from the 1930s to at least the 1960s.
Definitions
Webster's New World American Idioms Handbook (2011) says:
I'm down with that: slang; means “I'm agreeable to that”
A poster on Wordreference.com (2006) said:
In AAVE [African American Vernacular English] "I'm down" or "I'm down with that" can mean "I am in", I will commit to that action or position, or I am on your side.
Our Souls to Keep: Black/White Relations in America (1999) by George Henderson equates "I agree" with black English "right on" and "I'm down with that".
Law Enforcement Vocabulary (1973) by Julian A. Martin:
I'm Down With You. Juvenile slang: I'll fight on your side.
This version was used in Buffy The Vampire Slayer in 1998, showing overlap between being friends with someone or generally agreeable with them, and and described as mainstream slang in the book Slayer Slang: A Buffy the Vampire Slayer Lexicon (2004) by Michael Adams (first published online in 1999):
If Faith's Goth-chick slang veers towards the obscure, other characters favor the teen mainstream: ... "So, you're not down with Angel," she acknowledges of Spike, Angel's rival among Sunnydale vampires; ...
The American Heritage Dictionary of Idioms (1997) shows a slightly different use:
4. be or get down with. Be close friends with, as in I'm down with that crowd. [Slang; late 1900s]
From the introduction to A Jazz Lexicon (1964) by Robert S. Gold:
A few terms, perhaps because of their simplicity and widespread applicability, have survived from the early jazz life... The jazz slang speaker's aloofness is tacitly justified by his feeling that only those who are down with the action (aware of what is going on) should have access to the speech of those who have paid their dues (suffered an apprenticeship in life generally and in the jazz life in particular).
And its own entry (read online):
down with, [poss. from gambling slang to be down (i.e., to have one's bet placed) and poss. from general colloquial down to his toes (or socks); current esp. among Negro jazzmen since c. 1935] See 1957 and second 1959 quots. — 1944 Dan Burley's Original Handbook of Harlem Jive, p. 15. "I'm down with the action." — p. 41. Othello, the spade stud, pops in port, "down with it, cause he can't quit it." — p. 47. Iago is down with the action. — 1946 Really the Blues, p. 369. down with it: top-notch, superlative. — 1955 Down Beat, 5 Oct., p. 51. I don't know who the singer is, 'cause I'm not down with all the singers now. — 1957 The Book of Negro Folklore, p. 483. down with it: to get acquainted with, to understand. — 1959 Diggeth Thou?, p. 23. Let's see what's down with the deal. — 1959 Esquire, Nov., p. 70I. down with something, to be: to know something thoroughly. — 1960 Beat Jokes Bop Humor & Cool Cartoons, p. 57. The Ham wasn't down with the action.
Early examples
From an interview with rapper Guru from Gang Starr in Blues & Soul magazine (1990):
"You see I'm down with anything that's about uplifting and giving self-esteem to the black man. You see the black man in cities in America, over here and around the world . . . they need self-esteem because a lot of them see it as society's so hard on them they can't get it together. What these religions are based on is building pride, awareness and knowledge of culture and self, and that is important. So, anything like that I'm down with. That's why I say 'I'm down with the Nation [of Islam]"
Ebony magazine (January 1976):
Buffalo's Randy Smith, a former NSSFNS recipient, said he was determined to make the tournament, with or without his club's blessings. "That's how most of the players felt," said Smith. "The tournament is designed to help black youngsters, and anytime there is something I can do to help the cause, hey, I'm down with it."
New Black voices: an anthology of contemporary Afro-American literature (1972):
"Don't bet on that. Chumps like to deal with a winner, if you know what I mean. I'ma stone hustler. I'm down with it, dig? And he threatened me, even if it was with ...
From Mezz Mezzrow's autobiography Really the Blues (1946) , in a reference to jazz slang of the 1930s:
first cat: Hey there Poppa Mezz, is you anywhere?
me: Man I'm down with it, stickin' like a honky.
A translation of terms appears in the glossary:
Down with it: top-notch, superlative
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Best Answer
No one knows. The expression has existed in many languages for a long time, which suggests that its origin is pretty old.
There are several theories, some based in the similarities in many languages between the words inch and thumb and how you can measure an inch using the thumb, others based on the general usefulness of the thumb to measure different things.
It's entirely possible that it originally had nothing at all to do with the thumb; that it was a similar word that has become distorted over time, then translated to other languages in its distorted form.
See also: Rule of thumb on Wikipedia.