A dictionary of Confusable Phrases: More Than 10,000 Idioms and Collocations

let the cat out of the bag • let the genie out of the
bottle
let the cat out of the bag —(accidentally) reveal a secret: A
few days later, when I wasn’t around, she let the cat out of the
bag. Apparently, she didn’t realize she had ruined a surprise.
let the genie out of the bottle —let smth. bad happen that
cannot be put right or controlled: The ambassador warned
that the deployment of space weapons will “let the genie out
of the bottle.”
let the fox guard the henhouse • put the cat
among the pigeons
let the fox guard the henhouse —(also: set the fox to keep the
geese) trust a person with smth. that he is especially likely to
misappropriate or abuse: Back in 
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, members of the State
Department wrote the text of the treaty. Now different people
in the State Department are trying to define what was written.
I don’t believe we should let the fox guard the henhouse.
put the cat among the pigeons —(also: set the cat among the
pigeons) do or say smth. that is bound to cause trouble and
make people angry: I put the cat among the pigeons by rec-
ommending that revalidation should include the provision of
a certificate that the doctor has passed a knowledge test at
some stage in the five year period.
let’s be having you • thanks for having me
let’s be having you—is used at public places as a request to
move somewhere quickly: Ladies and gentlemen, let’s be hav-
ing you: the museum closes in ten minutes.
thanks for having me —thank you for your hospitality (said
as one says goodbye): Thanks for having me, Jo — we’ve all had
a lovely time!
letter box • mailbox
letter box —
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. (UK) a private box or other container to which
mail is delivered: When I came back from holiday the first
thing I found in my letter box was a letter refusing my pro-
posal for the TV show. 2. (UK) a narrow opening in a door
through which a mailman can push envelopes, cards, etc.: Mr.
Marble, who was already awake … heard the newspaper
pushed through the letter box of the front door downstairs.
3. a box in a public place in which letters are deposited for
dispatch: Of all the clues, the most promising of all was that
provided by the scrawl on a penny postcard, dropped in a
letter box in Newark. 4. a film format for movies when shown
on television: The term “letter box” was originally coined by
20th Century–Fox when CinemaScope first appeared in 
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953.
mailbox —
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. (U.S.) a private box to which mail is delivered
(in some areas the resident signals the mailman by putting up the
little red flag that the mailbox contains letters to be picked up and
mailed): A letter in his mailbox during his sophomore year of
college started Dawayne on a journey that took him to Boston,
then Hawaii. 2. = letter box 3: Letters are collected from mail-
boxes located throughout the country, each posted with a sign
that informs customers of mail pick-up times. 3. (Computers)
the directory where computer stores messages: How do I
change the display order of the messages in my mailbox? I
want the most recent items to appear at the top of my list.
See also: post box / post-office box.