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

know something by heart • know something in
one’s heart
know something by heart—be able to quote smth. from mem-
ory: Few lawyers know by heart the complicated statutes relat-
ing to Church matters.
know something in one’s heart—realize smth. in the depths
of one’s mind: Every soldier knows in his heart that he was
not made to fire upon the citizens.
knuckle down to something • knuckle under to
something
knuckle down to something —get down to work and do it
with great energy: This is a year of hard work when one must
knuckle down to the task at hand.
knuckle under to something —obey or yield to smth.: Rebecca
has shown great courage in refusing to knuckle under to this
pressure.
L
labor day • work day
labor day —
1
. (Labor Day) a day celebrated in honor of
workers, often as a public holiday: The governing committee
of the Stock Exchange decided to close on Monday, on account
of Labor Day. 2. (in the former U.S.S.R.) a unit of labor cal-
culated according to the expected productivity of a worker:
The amount of work which can be done in a day is worked
out for each farm operation and is counted as a “labor day.”
work day —(also: working day) 
1
. a day on which work is
usually done: This year, it happened that Christmas fell on
his work day. Still, it won’t keep his family from celebrating
with Santa. 2. the hours a person works: His work day would
generally last from lunchtime through dinner and then he’d
work until midnight at the least.
lack for something • lack in something
lack for something —have need of smth.; be without smth.:
Fish is so plentiful on this coast that the natives never lack for
food.
lack in something —fail to have enough of smth.: No one
who is lacking in musical ability will ever become a profes-
sional singer.