covert
back-door
backstairs
concealed
feline
fraudulent
furtive
hidden
hidlings
hole-and-corner
hugger-mugger
hush-hush
private
privy
quiet
secret
secretive
shifty
skulking
slinking
slinky
sly
sneaking
sneaky
stealthy
surreptitious
under-the-counter
under-the-table
undercover
underground
underhand
underhanded
unobtrusive
diplomatic
hasty
hazardous
illegal
illicit
improper
indirect
informal
large-scale
naval
ongoing
subsequent
unauthorized
destin
progestin