expression, godan verb (-ru)
God only knows; heaven knows; it's anyone's guess
A set phrase expressing that the outcome or truth is unknown and perhaps unknowable. Often used when the speaker feels that only a higher power could know the answer. The ぞ is an old-fashioned emphatic particle, giving the phrase a slightly literary or proverbial feel.
試験の結果は神のみぞ知る。
Only God knows the exam results.
この
The answer to this mystery is known only to God.