Quis custodiet ipsos custodes?

Mark Kleiman writes

This is an example of what Orwell calls a "dead metaphor," where the phrase has lost its original metaphorical meaning and become a pure idiom. ("Toe the line," which originally meant to stand properly in rank during military drill, is another such, as evidenced by the frequent misspelling "tow the line.") So how about using "restrain" or "control" or even "check" instead? Most of us don't ride horses.

This has been another official notice from the Language Police.

and helpfully links to the essay in which
In which George Orwell called "Toe the line" a dying metaphor and not a dead metaphor.

DYING METAPHORS. A newly invented metaphor assists thought by evoking a visual image, while on the other hand a metaphor which is technically ‘dead’ (e. g. iron resolution) has in effect reverted to being an ordinary word and can generally be used without loss of vividness. But in between these two classes there is a huge dump of worn-out metaphors which have lost all evocative power and are merely used because they save people the trouble of inventing phrases for themselves. Examples are: Ring the changes on, take up the cudgel for, toe the line, ride roughshod over, stand shoulder to shoulder with, play into the hands of, no axe to grind, grist to the mill, fishing in troubled waters, on the order of the day, Achilles’ heel, swan song, hotbed.

This has been an official notice from the Language Police's office of internal language affairs.