To give a horse “free rein” (also known as “giving him his head”) means to slacken off on the reins so the horse can run as fast as he likes in whatever direction he wants to go.
What a “free reign” might be is beyond my ken. It might, I suppose, refer to a constitutional monarchy, but there doesn’t seem to be any such idiom in English, and the phrase makes no sense in its usual context: “X gave Y free reign to Z.” For example, from Political Animal today:
I imagine Will has been around long enough that the Post‘s editors probably give him free reign to publish whatever he pleases.
The first few times I saw “free reign” I assumed it was just a typo that the spell-checker let through; but when as careful a writer as Steve Benen uses it I start to worry; there are almost as many Google hits for the wrong version as for the right version, though admittedly many of those refer to the band name.
Since very few of us ride horses these days, perhaps we ought to admit that “free rein” is a dead metaphor and give it a decent burial. But if it’s to be used, it could at least be used correctly.
See also: “tow the line.”
Update Turns out there’s a technical name for a mis-spelling or mis-pronunciation based on a folk etymology: “eggcorn,” from a distortion of “acorn.” (There’s even an “Eggcorn Database.” Other examples:
Straightjacket (for straitjacket)
Run the gauntlet (for run the gantlet)
Wait with baited breath (for “bated,” which is a short form of “abated”)
Damp squid (for “squib”: you’d think that an ocean-dweller would generally be damp, at least)
Front in center (for front and center)
It turns out that “curry favor” is originally an eggcorn. If you think about it, whether “curry” is used to mean “tend a horse’s coat” or “cook with hot Indian spices,” “curry favor” makes no sense. The original is “curry favel,” from a story in which a horse named Fauvel manages to dupe humans into brushing him down.
A special case is “grow like top seed” (for “Topsy”). Even the standard use of “grow like Topsy” to mean “grow quickly” or “grow out of control” is already a mistake. The reference is to Uncle Tom’s Cabin , where a slave-girl is being catechized by a white friend:
“Have you ever heard anything about God, Topsy?” The child looked bewildered, but grinned as usual. “Do you know who made you?” “Nobody, as I knows on,” said the child, with a short laugh. The idea appeared to amuse her considerably; for her eyes twinkled, and she added, “I spect I grow’d. Don’t think nobody never made me.”
So the correct use of “grow like Topsy” would refer to something that grew without plan, in good Hayekian fashion, not to rapid growth or to growth to great size.
The moral of the story: Make your own metaphors; if you must use them ready-made, at least be sure you can parse them.