guǐ guǐ suì suì

鬼鬼祟祟

sneaky; stealthy; secretive

Example:

两个人鬼鬼祟祟的, 不知在说什么。
liǎng gèrén guǐguǐsuìsuì de, bùzhī zài shuō shénme.
Goodness knows what they’re being so secretive about.

这家伙鬼鬼祟祟的, 想干什么?
Zhè jiāhuo guǐguǐsuìsuì de, xiǎng gàn shénme?
What’s that fellow up to, sneaking around like that?