Flatfoot Jack and the Fuzz Brigade.
Dear Word Detective: If you were able to get to the bottom of this one you would deserve a medal! In Australia, at least, and, I think, elsewhere, the police are referred to by criminals and other elements of society as “the Jacks.” Long hours of searching and asking questions of other sites has produced exactly zero. How can this be, when the word is so consistently used across the board? Perhaps if you cannot answer my first question, you can answer my second. — Aliki Pavlou
Medal, schmedal. Just send me one of those kangaroo things and a dozen sheep. The roo can do the dishes and the sheep can mow the lawn. They would also give Brownie the Dog (who claims to be part Border Collie) something more tractable to herd than the cats she’s been working with.
“Jacks” as slang for “police” is indeed common in the UK as well as in Australia, but virtually unknown in the US, although “Jacks” may have a close relative in US slang.
To begin at the beginning, “Jack” is what linguists call a hypocoristic (affectionate or “short”) form of the name “John,” derived from the French form of John, “Jacques.” As a slang term, “Jack” has assembled an impressive range of meanings, from “to jack up” (to increase, from the use of “jack” as a mock-personal name for a lifting mechanism) to “jack” meaning “nothing” (as in the eloquent double-negation “You don’t know jack about cars.”).
“Jack” as also been used, since at least the 16th century, as a stand-in for “the common man” or “a fellow,” as in “every man Jack needs a job.” The slang use of “Jack” specifically to mean “police officer” dates to the late 19th century (“A couple of men who were in plain clothes in the tap-room of a public-house, and were suspected by the ‘gaffer’ of being ‘Jacks’,” 1899).
This use of “Jack” to mean “police” seems to have been derived, again as a “short form,” from the use of “John” also as slang for “policeman,” and here things get interesting. This “John” was itself short for “John Darme” a joking Anglicization of “gendarme,” French for “police officer.” So “John Darme” became “John,” which became “Jack” as slang for “cop.”
But wait, it gets better. At least in Australia and New Zealand, “John Hop” was once also slang for “police” via rhyming slang, an underworld “secret language” where the phrase spoken rhymes with the hidden meaning. “John Hop,” of course, rhymes with, and signifies, “cop.” A contraction of “John Hop” (“jonnop”) is still current Australian slang for “police.”
In the US, “John” as slang for “cop” crops up only in “John Law” as the personification of the police and legal system (“We go mooching along the drag, with a sharp lamp out for John Law,” Jack London, 1906). It is possible that “John Law” harks back to the “John Darme” joke, but it may simply spring from the use of “John” in the US since the late 18th century as a personification of the average fellow (“John Q. Public,” etc.), a role now more often filled by “Joe” (as in “Joe Sixpack”).