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

Uncle Tom • Uncle Tom Cobley
Uncle Tom —(U.S.) a Black man who defers to white people
and regarded as betraying his social loyalty: I know exactly
what you think of me. Also, I know all the words, most of
them stupid — white nigger, Uncle Tom — as well as you.
Note:
The expression does not correlate in meaning with the
phrase Aunt Tom—(sl.) a woman who does not agree with or
support the Feminist movement: In a debate between Wolfgang
and Friedan, things rapidly devolved into mutual name calling
with each accusing the other of being an “Aunt Tom.”
Uncle Tom Cobley —(also: Uncle Tom Cobley and all) a whole
lot of other people (used at the end of a long list of names): We
are exhorted to pant along behind the industrious Germans,
Japanese, Russians, Americans and Uncle Tom Cobley.
under arms • under the gun
under arms —said of people who are armed and prepared
for fighting: Japan still had millions of troops under arms and
thousands of combat aircraft.
– 327 –
under arms

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under the gun — acting under a lot of pressure: They did a
great job, especially working under the gun with only a few
months to get it organized and executed .
Note: 
The expression does not correlate in meaning with the
phrase under the knife— used of smb. undergoing a surgical
operation: She was under the knife for several hours. When
she came out of surgery, doctors said she was fine.
under cover • undercover
under cover —
1
. under a shelter protecting from rain, wind,
etc: Monsieur had better come under cover. The “Bosches”
are still firing this way. 2. (of police or secret service agents) oper-
ating in secret: He worked under cover … in some of the most
important theaters of the Cold War. 3. (dated or formal) in an
envelope: The letter was passed to the Attorney General who
sent a copy of the letter under cover asking for comment. Cf.:
under plain cover — in an envelope which has no indication of
the firm, the contents, etc.: Credit card statements do not show
our company name and all products are shipped under plain
cover.