What does Mark 8:12 mean?
Jesus sighs deeply in indignation and grief. These scholars of the Law should have accepted the signs of fulfilled prophecies. But their hearts are hardened and their minds are not where God's is (Mark 8:33).The word "generation" has many possible meanings and causes much discussion when studying the New Testament. It can mean a genealogical level, but here it means a specific ideology held in a place and time. This "generation" includes the scribes (Matthew 12:38), and may include a crowd (Luke 11:29), although we don't know if the crowd follows Jesus or the Pharisees.
"Truly" is from the Greek root word amen. Literally, it means "a truth," and serves to bring attention to His words, like saying, "Know this for certain…" In Hebrew culture, however, it may be oath language (Numbers 5:19–22). Jesus is swearing that the religious leaders, who would rather test Him with foolish demands instead of studying how He fulfills the Messianic prophecies in the Old Testament, will not receive another sign except the sign of Jonah (Matthew 16:4), which He had mentioned before (Matthew 12:38–40).
Jesus adds that it's easy enough to determine the weather from the evidences given in the sky (Matthew 16:2–3). At another point, He warns that the Queen of Sheba and the men of Nineveh will judge the Pharisees of Galilee for their foolishness (Matthew 12:41–42). Nineveh and the Queen responded to Jonah and Solomon while the Pharisees, Sadducees, scribes, and Herodians ignore someone much greater.