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

Note: 
The expression does not correlate in meaning with the
phrase lift someone’s hair—(U.S. hist.) scalp a person: They
[Indians] wouldn’t hesitate to lift the hair of any lone white
man.
let someone down • let someone down easy
let someone down —disappoint a person; fail to support or
aid smb.: I leave it to you, Stephen. I know you’ll not let me
down.
Note: 
The expression is not antonymous in meaning to the
phrase let up on someone— treat a person less harshly or severely:
By the time Jon turned eight, our financial problems became
so overwhelming that I began to let up on him.
let someone down easy —(also: let someone down softly) treat
a person considerately so as to spare his self-respect: The fact
is, and I hope you would respect and appreciate this, I care
for him enough to let him down easy.
let someone in for something • let someone in on
something
let someone in for something —involve a person in smth.
unpleasant; cause smb. to have trouble: The boy’s careless
treatment of the car has let me in for a large repair bill.
Note: 
The expression does not correlate in meaning with the
phrase put someone in for something— enter a person for an
examination, contest, etc.: How many of your runners are you
putting in for the race?
let someone in on something —make smth. such as a plan,
agreement, etc. known to a person: He was let in on the
arrangements without our consent.
let someone off • let someone off the hook
let someone off —excuse a person; not punish severely
enough: “Are you going to let him off?” demanded Denny
suspiciously. “You never can be stiff in the back, Charley.”
let someone off the hook —(also: get someone off the hook) free
a person from difficulty, an embarrassing situation, etc.: The
officer was pretty surprised at my tenacity, and it motivated
him to let me off the hook.