mark 8:12 And he sighed deeply in his spirit, and saith, Why doth this generation seek after a sign? verily I say unto you, There shall no sign be given unto this generation. - Free Bible Online

      12. And he sighed deeply in his spirit--The language is very strong. These glimpses into the interior of the Redeemer's heart, in which our Evangelist abounds, are more precious than rubies. The state of the Pharisaic heart, which prompted this desire for a fresh sign, went to His very soul.
      and saith, Why doth this generation--"this wicked and adulterous generation" (Mt 16:4).
      seek after a sign?--when they have had such abundant evidence already.
      There shall no sign be given unto this generation--literally, "If there shall be given to this generation a sign"; a Jewish way of expressing a solemn and peremptory determination to the contrary (compare Heb 4:5; Ps 95:11, Margin). "A generation incapable of appreciating such demonstrations shall not be gratified with them." In Mt 16:4 He added, "but the sign of the prophet Jonas." (See on Mt 12:39, 40.)

JFB.