A compinsult, used when someone is smarter than they appear. It may be that it's being looked at such that the "thing" (whatever it is) is deceptive. In a way, that's true, but it is we who are being deceived. If his intelligence is deceptive, I am being deceived as to his intelligence.
mike: you are deceptively intelligent
mary: what is that last word mean that starts with a d
mike: look it up
mary: so ur telling me that im lying to u about what im telling u
mike: no, mary, try again...