Adjective: furtive, deceitful, unfair, or sly.1)

It is also a defining characteristic of certain bastards that inhabit Improbable Island. Should you meet a sneaky person, place, or object2)3), the only option is to run screaming into the night4)

1)
Also rumored to sometimes be used to describe something that is impossible to kill.
2)
Especially if it's a sneakily moving box.
3)
and even more especially if a big question mark or exclamation point appears over your head when you go to examine said box.
4)
Unless it's day, in which case you're screwed.