secret of Polichinelle
top secret
mum
crypto-
pseudosecret
closehanded
under the covers
on the down-low
on the hush
out with it
off-the-record
dark
subterranean
something nasty in the woodshed
ultrasecret
let something slip
secretly
out
let the cat out of the bag
tomb
covert
under the table
divulgation
semisecretive
make known
occulted
secretively
give the game away
underhanded
in petto
secernent
clandestinely
under the rose
private
secretomal
shoot off at the mouth
confidentiality
spill the beans
air one's dirty laundry in public
repository
backstage
exposure
whisperer
underhand
epopt
skeleton in the cupboard
confidence
disclosed
supersecret
covertly