adumbrate
Meaning: to sketch out in a vague way
Usage: The coach adumbrated a game plan, but none of the players knew precisely what to do.
enigmatic
Meaning: mystifying, cryptic
Usage: That man wearing the dark suit and dark glasses is so enigmatic that no one even knows his name.
esoteric
Meaning: understood by only a select few
Usage: Even the most advanced students cannot understand the physicist’s esoteric theories.
nebulous
Meaning: vaguely defined, cloudy
Usage: The transition between governments meant that who was actually in charge was a nebulous matter.
uncanny
Meaning: of supernatural character or origin
Usage: Luka had an uncanny ability to know what others were about to say.
amorphous
Meaning: without definite shape or type
Usage: The effort was doomed from the start, because the reasons behind it were so amorphous and hard to pin down.