hugger-mugger
closehandedness
mum
on the QT
secretly
on the Q.T.
in secret
darkness
semisecrecy
down low
cloakedly
semisecret
under one's hat
hush-hush
hidness
secretively
hypersecrecy
secretive
semisecretly
clandescent
secernent
private
confidentiality
underhand
closehanded
backroom
furtive
subterranean
closeness
backstage
confidence
confidentialised
dark
sub rosa
under the covers
on the down-low
tightlipped
closemouthed
privily
closelipped
close
a man's home is his castle
hypersecretory
under the table
pent
confidentially
hole-and-corner
sanctum sanctorum
under wraps
on the hush