Neither fluff, nor feather. – (Go) to devil! ~ Russian Proverb
This weird expression, which actually means “Good luck” came to russian speech from hunters. It is based on the superstitious notion that wishing for the desired outcome of the hunt (catching “fluff or feather”), the hunt can be “jinxed.” People are afraid of jinxing even today, so it became a wish for good luck. You can say “Neither fluff, nor feather” to the student before an exam or in any other risky situation.