surreptitious (adj.)

• artful
• backstairs
• calculating
• clandestine
• collusive
• concealed
• covert
• crafty
• cunning
• deceitful
• false
• feline
• fraudulent
• furtive
• guileful
• hidden
• hole-and-corner
• hugger-mugger
• hush-hush
• indirect
• insidious
• private
• privy
• quiet
• scheming
• secret
• secretive
• sharp
• shifty
• slippery
• sly
• sneaking
• sneaky
• stealthy
• treacherous
• tricky
• two-faced
• under-the-counter
• under-the-table
• undercover
• underground
• underhand
• underhanded
• unobtrusive
• veiled
• wily