A.T. Robertson Commentary Matthew 12:39

"But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given it but the sign of Jonah the prophet:" — Matthew 12:39 (ASV)

An evil and adulterous generation (γενεα πονηρα κα μοιχαλις). They had broken the marriage tie which bound them to Jehovah (Plummer). See Ps 73:27; Isaiah 57:3ff.; 62:5; Ezekiel 23:27; James 4:4; Revelation 2:20. What is "the sign of Jonah?"