Does anybody know the source of this idiom or have an explanation of how it originated? I know it means that the speaker does not trust the person in question, but I want to know the etymology of the idiom. How did it mutate from something literal into this? Obviously trust is not measured in the same way as distance, so I want to understand the correlation in this context.
I can't offer an authoritive source. However there is another expression, 'I can't trust him out of my sight' that makes the meaning obvious.
Most children go through a stage where their parents must keep a careful eye on them to prevent the child getting up to mischief. The parent might say, "Now he is a toddler, I can't trust him of out of my sight. He is always poking his fingers into everything and I'm worried he may electrocute himself or pull the bookshelves on top of him." This literally means that the parent is happy only when they can see the child.
When you can trust someone as far as you can throw them, it is just an exaggerated form of the previous expression. It means we can trust them at zero distance, i.e. not at all. We usually say this about adults and of course most people cannot throw another adult any distance at all.
I've found some more evidence as to how the phrase may have developed.
If you haven't got any confidence in a man, you can't say much worse of him than this — ' I'll trust him as far as I can see.'
Ye cain't trust him as far as ye kin throw a b'ar. That's why we call him Snake.
I know all about Otto Ernsthausen. I wouldn't trust him as far as I can spit!
However A Dictionary of Catch Phrases By Eric Partridge traces it back to 1870.
P.P.S. I think I've beaten Eric Partridge with this !
Somethin furthur,' siz I, ' than I'd trust you.' 'How far is that?' siz he. “Just as far, then, siz I, as I could throw a bull by the tail. The Westminster Review - Volume 9 - Page 434 - 1828
Its origin appears to be obscure as suggested by the Phrase Finder, (also the use of the initial negation is a matter of debate):
(informal) Not trust or hardly trust a particular person at all.
- I would not trust that guy as far as I can throw him, and well, I am small and not strong- so I hope that makes my point.* (ODO)
Origin of "trust him as far as I can throw him":
It's an answer to the question, "How far do you trust this fellow?" Answer: "I trust him as far as I can throw him." Since even a strong man can scarcely throw another full-grown man any further than straight down, the answer is tantamount to "I don't trust him at all.
" Who first said this? I imagine that that is lost in the mists of time--but not very much time, as I imagine the expression is not much more than a century old, if that.
If you search the expression in Google, you will discover something very odd. A large proportion of those using the expression use it incorrectly, as "I DON'T trust him as far as I can throw him," thus depriving the sentence both of meaning and of the sardonic wit that inspired it.
In Nicholas Nickleby (Chapter 17, pg 210), published 1838/1839, Charles Dickens describes Mrs. Knag as one of a [class of people] "described by the axiom, that you can trust them as far as you can see them, and no farther." Hence, it seems that this predates the 1874 reference to the City Road Magazine by 35 years!