Synonyms and related words :
back-door, backstairs, covert, feline, furtive, hidlings, hole-and-corner, hugger-mugger, privy, quiet, shifty, skulking, slinking, slinky, sly, sneaking, sneaky, stealthy, surreptitious, under-the-counter, under-the-table, undercover, underground, underhand, underhanded, unobtrusive,