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

Note: 
Neither expression correlates in meaning with the phrase
let something go—pay no attention to smth.; neglect smth.:
Robert let his teeth go when he was young and now he has to
go to the dentist pretty often.
let it all hang out • let it go hang
let it all hang out—(coll.) let the truth be known; not dis-
guise anything: No names, of course, will be used; he doesn’t
expect everyone will be as willing as he is to let it all hang out.
Note: 
The expression does not correlate in meaning with the
phrase hang it all—(exclam.) used to express one’s anger, irri-
tation, etc. (also: hang it): “Oh, hang it all, Kit, why don’t you
say something?”
let it go hang —
1
. (coll.) become indifferent to smth.; take
no interest in smth.: Either you work on pronunciation very
well, or you just let it go hang. 2. (exclam.) is used to express
one’s anger or irritation with smth.: Love was the only thing
in the world that mattered. Achievement — pah! Let it go hang!
let off steam • put on steam
let off steam —(also: blow off steam) give vent to one’s
repressed emotions (by talking in an unrestrained manner,
etc.): This special session will give politicians a chance to let
off steam.
put on steam —(also: get up steam) build up strength or
energy; become more active: Get on a little faster; put a little
more steam on, ma’am, pray.
let one’s hair down • put one’s hair up
let one’s hair down —(also: let one’s back hair down) throw
off restraint in one’s behavior: Those who are doomed to a life
of formality look forward to the week-end when they can let
their hair down.
put one’s hair up —assume adult responsibilities: Lively
young thing, I recall — but she’ll have put her hair up by now.